Settings ⌘K search: tab state and URL diverge, then a search silently does nothing

[Optic00]

Follow-up from the #399 review (merged while the review was being finalised).

The Settings tab state and the URL can diverge, after which a ⌘K settings search silently does nothing:

1. ⌘K → "AI provider" → navigates to /settings?tab=ai (works)
2. Click "General" in the nav rail → SettingsNav onSelect={setTab} changes only local state; the URL still says ?tab=ai
3. ⌘K → "AI provider" again → navigate('/settings?tab=ai') bails because the hash is unchanged (router.ts early-return), the route effect never fires, the palette closes, and the visible tab stays General.

Fix directions: make the route the single source of truth (nav-rail clicks call navigate('/settings?tab=<id>') and the existing route effect switches the tab), or clear/normalise the param on manual tab switches. The T1 spec covers a distinct hash transition only - the search-after-manual-switch sequence above is the regression test to add.

Two smaller leftovers from the same review:

• SETTINGS_INDEX lists macOS-only settings unconditionally ("Record system audio", "Hide dock icon" are behind isMac in GeneralTab) - on Windows they navigate to a tab where the row doesn't exist. Filter on isMac.
• "Discord" (AboutTab) and "Show in menu bar / system tray" (GeneralTab) are still missing from the index; a T1 assertion that each index title matches a rendered label would stop the drift that already needed one fix commit.