#5463 deleted `pages/ChannelsPage.tsx` wholesale on the assumption
that "every interactive path on the page is broken, so the page
itself is broken." That's not the right conclusion: the four
surviving channel endpoints (`GET /api/channels`,
`POST /api/channels/reload`, `GET /api/channels/registry`,
`POST /api/channels/sidecar/{name}/configure`) are exactly what a
channels-management dashboard needs, and the page already had a
schema-driven `SidecarForm` drawer wired to them. Dropping the
page forced channel configuration out of the UI and back into
hand-editing `config.toml`, which is a meaningful UX regression.

This PR restores the page in its sidecar-only form (the refactor
that #5463 should have shipped instead of deletion):

  - `pages/ChannelsPage.tsx` (770 lines, was 1488 pre-#5463) —
    drops `ChannelForm` / `InstancesDialog` / `QrLoginDialog`
    wholesale (each drove a deleted endpoint cluster). Trims
    `DetailsModal` to the sidecar branch only (read-only "manage
    via config.toml" note + copyable `config_template`). Body
    drops `testMut` / `handleTest` / `configuringChannel` /
    `qrLoginChannel` / `closeQrLogin` / the qr & legacy branches
    in `handleCardConfigure` and `handlePick` — every channel is
    sidecar now, so both fall straight through to
    `setSidecarFormChannel(ch)`.

  - `pages/ChannelsPage.test.tsx` (290 lines, was 524) — covers
    the 9 surviving flows: loading skeleton, empty-state CTA,
    configured-list render, search filter, picker drawer with
    unconfigured channels, picker → sidecar configure drawer
    swap (single-commit), schema-driven Save → `useSaveSidecarConfig.mutate`,
    Reload header button → `useReloadChannels.mutate`, and the
    details-modal `config_template` surface. The 13
    instance / test / qr test cases that #5463 retired stay
    retired.

  - `api.ts` — restores ONLY the `ChannelField` type
    (consumed by `ChannelItem.fields` + `SidecarForm`). The 6
    dead stubs / 4 QR helpers / `ChannelInstance` /
    `ChannelInstancesResponse` / `QrStartResponse` /
    `QrStatusResponse` stay gone.

  - `lib/queries/channels.ts` — same: only `useChannels` /
    `useCommsTopology` / `useCommsEvents`. `useChannelInstances`
    stays gone.

  - `lib/mutations/channels.ts` — same: only `useReloadChannels`
    / `useSaveSidecarConfig` / `useSendCommsMessage` /
    `usePostCommsTask`. The 5 wrapper-of-deleted-endpoint hooks
    stay gone.

  - `lib/queries/keys.ts`, `lib/http/client.ts` — surface only
    what survives.

  - Page wiring restored: `/channels` route + `lazyWithReload`
    entry in `router.tsx`, route-type union + nav entry (and
    `Network` lucide import) in `App.tsx`, `n: { to: "/channels" }`
    vim shortcut in `useKeyboardShortcuts.ts`, `channels`
    navigate-to entry in `CommandPalette.tsx`. The
    `librefang.ai/channels` REGISTRY_ITEMS entry was always
    there and is not touched.

  - i18n restored: `nav.channels` in both `locales/en.json` /
    `zh.json`, plus the page-specific `channels.*` namespace
    (now 32 keys, down from 48 — the deleted-feature keys
    (`test_success` / `test_failed` / `qr_login` / `qr_failed` /
    `login_success` / `update_config` / `no_instances` /
    `add_instance` …) are not restored).

  - Stale-doc-comment fixes from #5463 are reverted in the four
    pages that referenced `ChannelsPage` (the cross-reference is
    valid again now that the page exists).

Verification
------------

  pnpm typecheck                            → clean
  npx vitest run src/pages/ChannelsPage.test.tsx → 9 passed

Branched off main directly; not stacked on the now-merged
#5463 branch.

Self-review of the initial restore PR turned up a real broken test,
a dead UI branch, an inaccurate CHANGELOG, and dead-code regrowth.
This commit closes all of them.

Fixes
-----

  - SidecarForm no longer opens with a blank form when the operator
    re-configures an existing channel. Non-secret fields with a
    populated `value` pre-fill from the schema; secret-typed fields
    with `has_value: true` render a "•••• (set — leave blank to keep)"
    placeholder so the operator knows the slot is filled and won't
    wipe it by submitting blank. handleSubmit's "drop empty values
    from payload" rule is preserved — server treats absent keys as
    "leave alone," so partial-update flows are safe.

  - SidecarForm now exposes the row's `config_template` TOML snippet
    under a <details> summary at the bottom of the drawer. Closes the
    "snippet path is unreachable" gap from the review: previously the
    `config_template` field on ChannelItem was only displayed by an
    `!configured && config_template` branch in DetailsModal that was
    unreachable (configured cards never pass that guard; unconfigured
    discovery rows only surface in the picker, which routes to
    SidecarForm rather than DetailsModal). Moving the snippet onto
    SidecarForm lets ops who prefer hand-editing copy the canonical
    entry without leaving the configure flow.

  - DetailsModal trimmed to the actually-emitted, actually-useful
    fields. Removed: the `difficulty` / `setup_time` / `setup_type`
    rows (backend emits "" / "" / "sidecar" for every sidecar row —
    "" doesn't render under the conditional guards and "sidecar" is
    redundant signal on a sidecar-only page), the `webhook_endpoint`
    block (sidecars run their own webhook server, field is never
    emitted), the `setup_steps` block (always the same two-line
    boilerplate that duplicated the "manage via config.toml" note
    below it), and the unreachable `!configured && config_template`
    snippet branch (moved to SidecarForm per above).

  - ChannelItem shrinks from 14 fields (post-cherry-pick) to 8:
    dropped `instance_count` (always 0 or 1 for sidecars), `difficulty`
    / `setup_time` / `quick_setup` / `setup_type` / `setup_steps`
    (in-process metadata from the deleted CHANNEL_REGISTRY), and
    `webhook_endpoint` (never emitted post-sidecar). Kept `has_token`
    / `fields` / `config_template` — all three are real signal the
    page consumes.

  - ChannelCard drops the `instance_count_short` chip (cards always
    showed 1x / 0x for sidecars, no signal).

  - Test that claimed to "open the details modal with the config
    template" was deleted — the title promised a path the page could
    not reach, and the assertions only checked picker drawer
    visibility, not the DetailsModal. Replaced with three real tests:

      - "pre-populates non-secret field values from the sidecar
        schema" — asserts f.value populates the Input's displayValue.
      - "uses a 'currently set' placeholder for secret-typed fields
        with has_value" — asserts the i18n-keyed placeholder appears
        on the masked Input.
      - "offers the copyable config_template snippet inside the
        SidecarForm drawer" — asserts the <details> summary text and
        the [[sidecar_channels]] snippet body are both in the DOM
        (queryable regardless of open/closed state).

    Total: 11/11 passing.

  - i18n `channels` namespace pruned from 48 to 27 keys: every key
    that drove a deleted endpoint or in-process-only feature is gone
    (test_success, test_failed, qr_login, qr_failed, login_success,
    update_config, no_instances, add_instance, config_key, config_value,
    no_unconfigured, config_failed, config_success, no_fields_required,
    setup_adapter, difficulty, setup_time, setup_type, setup_steps,
    instance_count_short, …). Added: `secret_set_placeholder` and
    `config_template_summary` for the two new t() calls. i18n-parity
    script confirms en + zh aligned at 3473 total keys.

  - Stale doc-comment fixes:
    - DrawerPanel.test.tsx:140 — no longer references
      ChannelsPage::ConfigDialog (the component never existed in the
      restored page); rewritten to "same shape as ChannelsPage's
      SidecarForm drawer."
    - ProvidersPage.tsx:1511-1512 — drops the fictional "Mirrors the
      ChannelsPage empty state introduced when its tabs were retired"
      — the restored ChannelsPage never had tabs.

  - lib/queries/channels.ts reset to main's content — the multi-line
    -> single-line import that the cherry-pick produced was pure
    formatting noise, no semantic change.

  - CHANGELOG entry rewritten with accurate numbers: page 761 lines
    (was claimed 770), test 386 lines / 11 cases (was claimed 290 /
    9), namespace 27 keys (was claimed 32 — that was wrong; the
    cherry-pick brought back all 48).

Verification
------------

  pnpm typecheck                                  -> clean
  npx vitest run src/pages/ChannelsPage.test.tsx  -> 11 passed
  node scripts/i18n-parity.mjs                    -> parity (3473 keys)