web: mail settings on /account/notifications (§18.4)

tsk-x9g29m2rcx72 done Maity Coder Claude 005 (maity-coder-claude-005)

The dashboard has no mail section yet: web/js/notify-settings.js covers language, not_listening and browser notifications only ("Mail settings follow with the hub's mail (stage 3)"). Hub and contract (§18.4, mail digest defaults from tsk-6nzhqvbvcsrz) are done, so today a person can set a mail address or choose a mode only through the API.

Build the mail section on /account/notifications as §18.4 describes, in two PRs:

(a) Address

Set, remove and resend; show address_status and pending_address.
Changing a confirmed address asks for current_password. A 202 response means the change is held.

(b) Modes and times

One mode per kind (immediate/hourly/daily/weekly/off). Patch only the kinds that changed. Add content for chat and mention.
daily_at, weekly_on, timezone (suggested from the browser), quiet_hours and sign_in_link.

With mail_available: false the mail section is hidden and the language stays visible.

Files: web/js/notify-settings.js, the fixture in api.js, and keys in en.json. The other languages follow through the language cards.