fix(settings): reset to General on bare /settings so browser Back can't strand a stale tab

[Optic00]

Follow-up to #411 (which closed #405): one edge of the tab/URL divergence is still reachable.

The leftover bug

#411 made the route the single source of truth - nav-rail clicks now navigate /settings?tab=<id> and push entries onto the hash history. But the route->tab sync effect only reacts to a valid ?tab= param and ignores a bare /settings. So:

1. Open Settings (bare /settings, General visible).
2. Click any other tab in the nav rail (URL becomes ?tab=ai).
3. Press browser Back - the URL returns to bare /settings, but the AI tab stays visible. Tab and URL have diverged again, and a subsequent navigate('/settings') from anywhere bails on the unchanged hash, so nothing ever corrects it.

Fix

• One shared tabFromRoute resolver used by both the first-mount initialTab and the route-reactive sync effect, so their semantics can't drift.
• The sync effect now treats an absent (or unknown) ?tab= the same way first mount does: reset to General.

Test

New T1 regression in settings-cmdk-search.t1.spec.ts: nav to AI via the rail, browser Back, assert the hash is exactly #/settings and General is visible again. Verified the test fails on current main and passes with the fix.

Verification

• typecheck:renderer clean; lint:renderer identical warning count to main (35/35, no new findings).
• Full settings-cmdk-search T1 spec: 5/5 passing (including fix(settings): ⌘K search tab-state/URL divergence + index gaps (closes #405) #411's regression test).
• Cross-family review (Codex, read-only): SHIP, no blocking findings; its test-polish suggestion (exact-hash assert) is included.

---

Summary by cubic

Fix Settings tab desync when going Back to bare /settings. If ?tab= is missing, the visible tab now resets to General so the tab and URL stay in sync after nav-rail clicks.

• Bug Fixes
  • Added shared tabFromRoute resolver used by both the initial tab and the route-sync effect to keep behavior aligned.
  • Route-sync treats absent or unknown ?tab= as General, fixing the leftover edge case from Settings ⌘K search: tab state and URL diverge, then a search silently does nothing #405 after fix(settings): ⌘K search tab-state/URL divergence + index gaps (closes #405) #411.
  • Added a T1 test that navigates to AI, goes Back, asserts #/settings, and verifies General content is shown.

^{Written for commit 1c30d62. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{You’re at about 92% of the monthly reviewed-line limit. You may want to disable incremental reviews to conserve quota. Reviews will continue until that limit is exceeded. If you need help avoiding interruptions, please contact contact@cubic.dev.}

_{Re-trigger cubic}