Refresh and the detail view now follow a single preview target
instead of peeking at the list cursor or the detail's own state.
This keeps the right-hand views coherent when the preview
diverges from the list selection. Previewing a sub-issue is the
motivating case.

Switching the info panel to the Subtasks or Links tab previews
the tab's first entry right away, and moving the cursor through
the list updates the preview as you go. The Fields tab stays
quiet: navigation there never changes the preview target, and
empty lists dispatch nothing.

Preview requests debounce for 150 ms before hitting the network,
so scrolling through sub-tasks or links does not trigger a fetch
per row. A monotonic request counter rides along with each
fetch; any response whose counter is stale by the time it
arrives is dropped, so a slow answer for an earlier row cannot
overwrite a later selection. Bubbletea has no native command
cancellation, so the counter acts as the cancel substitute.

Cache hits short-circuit the debounce entirely and update the
detail view synchronously, so revisiting an already-loaded issue
feels instant. Cache misses land on the debounced path.

Non-preview paths (refresh, main-list selection on cache miss,
comment updates) keep the existing direct-fetch behavior.

Deletes the issueCache entry for the previewed key synchronously in the
ActRefresh handler, before the fetch cmd is returned. Any cache read
between the keypress and the fetch response now sees a miss instead of
stale data.

Adds TestActRefresh_InvalidatesCacheBeforeFetch to pin the invariant.

When the user presses Esc (ActFocusLeft) while the InfoPanel has focus
with the Sub or Lnk tab active, previewKey now snaps back to the
current list selection via previewSelectedIssue(). The same reset fires
when switching the InfoPanel tab to Fields via PrevTab or NextTab.

Empty Sub/Lnk lists never dispatch PreviewRequestMsg (confirmed by
test; no production change needed).

Added package-level subKey1 constant and migrated all "SUB-1" literals
in pkg/tui tests to use it (goconst lint fix).

Previewing a sub-issue now only updates the right-side detail
view. The info panel keeps its tab and cursor on the main list
issue, as the user's navigation context should.

Before, a detail response for a previewed sub-issue overwrote
the info panel. Because the panel's SetIssue resets cursor and
active tab when the key changes, the sub/link tab snapped back
to fields and the cursor jumped to the top, making the whole
pane feel like it was flying away from under the user.

The detail view and info panel now have independent guards.
The detail view follows previewKey, which is the source of
truth for what is shown on the right. The info panel follows
the list selection, matching user intent for navigation
context.

Selecting a task in the list, and switching back into the list
view, fires a single JQL batch for the task's parent, subtasks
and linked issues. By the time the user moves into the Sub or
Lnk tab, those issues are already in the cache and the preview
renders without a network round-trip. The parent joins the
batch so future drill-up navigation benefits from the same
warm cache.

Edit, copy URL, open in browser, transition, priority, assignee,
duplicate, create branch, and custom commands all now act on
the issue currently shown in the detail view. Previously they
followed the list cursor, which surprised the user after
drilling into a sub or linked issue via the info panel.

A new currentIssue helper resolves to the cached previewed
issue when one is active, falling back to the list selection
otherwise. The info panel's Fields tab still edits the main
list issue, because that is what the Fields tab shows.