feat(settings): restructure notification settings UX (#2644)

* feat(settings): restructure notification settings UX

Delivery Mode selector moved to top as the primary choice (real-time
vs digest). Real-time section (enable toggle, sensitivity, quiet hours)
only shows when real-time is selected. Digest section (send-at hour)
only shows when a digest mode is selected. Single shared timezone
picker replaces two separate ones. Removes UX confusion about whether
digest requires the enable-notifications toggle.

No API or schema changes. Both quietHoursTimezone and digestTimezone
are written from the single picker.

* fix(settings): auto-enable on digest switch, fix timezone init priority

P1: Switching to digest mode now auto-enables the alert rule (the
enable toggle is hidden in digest mode, so users couldn't turn it on).
Without this, digests were silently disabled.

P2: Shared timezone now prefers digestTimezone when in digest mode and
quietHoursTimezone when in real-time, using raw alertRule fields instead
of pre-defaulted variables that always resolved to detectedTz.