Follow-up to #3563, closing out the design-review backlog that PR deliberately
deferred. Tracked as lotti3-t4m.8.

Undo a link you just made

The highest-value item left over. Picking a candidate row commits on that
single tap — deliberately, since the speed is the point of the flow — but there
was no recovery: a mis-tap, or the right task under the wrong relationship,
silently persisted a real edge whose only removal path was the card's manage
mode.

This does not add a confirm step. It adds an undo:

• On success, a message names what was actually written — Blocks: Draft crew rotation schedule — rather than a generic "linked".
• Undo removes exactly the (fromId, toId, type) triple just created, so it
  can only take back the edge the message is about, never another relationship
  the same pair also holds (which Redesign task relationship links: one directed choice, editable in place #3563 made possible).
• A rejected link (cycle guard) shows the rejection and offers no undo.

Two experts and two personas raised this across review rounds 3-6; the
low-vision and novice personas both scored it an outright blocker, and it was
the largest single contributor to the gap between the panel's 7.17/7.0 and its
8/10 bar.

Manage mode says it is on

Manage mode was visible only as two icons appearing per row, and the way out
was the same unlabelled overflow menu it was entered from. The header now
swaps its link action for an inline Done while the mode is active, and the
overflow offers "Manage links…" only as a way in — so the menu item stops
being a second control for the same state.

Smaller items from the same backlog

• The picker search field autofocuses on desktop-width windows only. On a
  phone it would raise the keyboard over the very results it filters.
• The linked-tasks overflow menu uses the app's own surface colour and radius
  tokens instead of Material scheme defaults, elevation: 8 and a raw
  circular(12) — it was a themed island inside the card it opens from.

Verified, not assumed

One backlog item turned out to be based on a wrong premise and is not
"fixed" here: completed tasks are not offered as link candidates. The query
already passes openTaskStatuses, which excludes DONE and REJECTED. The
finding came from tests that stub getTasks to return done/rejected tasks
regardless of the filter argument, which made it look reachable in the UI.

Still deferred, with reasons

• FTS5 matches beyond the 200-row prefetch. Search filters an in-memory
  prefetch, so a match outside that window is invisible. Fixing it properly
  means querying by match rather than filtering a page, which is a data-layer
  change, not UI.
• Create-from-search in the picker. EntityPickerSheet already supports
  createFromQuery; wiring it would let the overflow's "Create new linked
  task…" and the last un-migrated AlertDialog collapse into one flow. That is
  a flow redesign worth its own review, not a rider here.
• Reconciling DesignSystemDropdown's field surface with
  DesignSystemSearch's. Genuine inconsistency, but a design-system-level
  decision affecting every caller of both.

Testing

• New: undo removes exactly the created triple (verified against the
  repository call, not just the SnackBar); a rejected link offers no undo;
  manage mode's inline exit returns to browse mode; entryLinkTypeDbName
  pinned against every EntryLinkType variant.
• flutter analyze clean project-wide.
• Full patch coverage on new/changed lines.

Version bumped to 0.9.1064+4244 with its own CHANGELOG and flatpak entries.