Hide the sidebar update control unless an update is available

[felipeorlando]

Closes #8

The automatic update probe moves from SidebarUpdate up into SidebarUpdateFooter, so the footer owns the snapshot and can decide whether anything is worth rendering. New exported predicate isSidebarUpdateActionable(snapshot) returns true only for available and downloading; the footer returns null otherwise, so the wrapper padding disappears too. SidebarUpdate is now presentational — the dead "Check for updates" / "Checking…" labels, the RefreshCw icon and the runUpdateFlow import are gone. A failed probe lands on idle, so it stays silent.

Settings and app-menu manual checks keep their own snapshots and runUpdateFlow(true, …), untouched. UpdateRailCard and the post-install "What's new" card are unaffected.

Judgement call to review

The Settings block was p-2 pt-0, relying on the footer above for its top gap. With the footer now able to vanish, Settings would sit flush against the scrolling session list. Spacing moved onto the Settings block (p-2) and the footer became p-2 pb-0, in both Sidebar.tsx and ProjectRail.tsx.

Verification

npm run check:web — exit 0, 128 files / 1328 tests. tsc --noEmit clean. 5 new tests in src/chrome/SidebarUpdate.test.ts.

Not run in the app — layout of the collapsed footer is unverified visually.

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