M4 Calendar: reminders form, invitations, short publish links, settings UX

Calendar settings (Settings > Calendars):
- Reminders are now {method, minutes}: Add-reminder opens a boxed form with a
  method dropdown (Show an alert / Email me) + time dropdown, bell icons.
  Backend normalizes the legacy list[int] transparently.
- New Invitations section (auto-add emailed events): policy radios, "Add new
  events from" (My contacts / Anyone), "Add to calendar", mark-read+file toggle.
  Persisted to user settings (inbound iMIP processing still deferred).
- Time zone options show GMT codes, e.g. "Central European Summer Time (GMT+2)".
- Default calendar now defaults to "Personal" instead of a generic placeholder.
- Colour picker is a circular-swatch-only dropdown (sized so swatches don't overlap).
- Delete calendar / remove subscription now confirm via a modal.
- Create form: dropped the breadcrumb; heading stays "Create calendar" even after
  a Publish toggle auto-creates the calendar.

Publish links: replaced the 244-char vault-sealed token with a 16-char random
slug (secrets.token_urlsafe). New sdk.public_session() (privileged, non-RLS) lets
the public /public/{token} feed resolve the slug directly with explicit filters.

Settings shell: "<- Mail" is now a context-aware "<- Back" via ?from=; Calendar
opens Settings with from=/p/calendar so Back returns to the calendar.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U72FZiF1W1pb8XxyWBTREA

frontend/src/i18n/en.ts

@@ -38,6 +38,7 @@ export const en = {
   "auth.register": "Register",
 
   "nav.mail": "Mail",
+  "nav.back": "Back",
   "nav.settings": "Settings",
   "nav.compose": "Compose",
   "nav.collapseSidebar": "Collapse sidebar",
@@ -254,6 +255,23 @@ export const en = {
   "settings.subscribed": "Subscribed",
   "settings.comingSoon": "Coming soon",
   "settings.subscribeError": "Couldn't subscribe to that URL.",
+  "settings.removeSubscription": "Remove subscription",
+  "settings.removeSubscriptionConfirm": "Stop syncing “{name}” and remove its events?",
+  "settings.deleteCalendar": "Delete calendar",
+  "settings.thisCalendar": "this calendar",
+  "settings.deleteCalendarConfirm": "Delete “{name}” and all of its events? This can't be undone.",
+  "settings.showAlert": "Show an alert",
+  "settings.emailMe": "Email me",
+  "settings.invitations": "Invitations",
+  "settings.inviteWhen": "When I receive events by email",
+  "settings.inviteAddUpdate": "Add new events to my calendar and update events that have changed",
+  "settings.inviteUpdateOnly": "Only update events that I have already added to my calendar",
+  "settings.inviteNone": "Do not automatically update my calendar",
+  "settings.addNewFrom": "Add new events from",
+  "settings.myContacts": "My contacts",
+  "settings.anyone": "Anyone",
+  "settings.addToCalendar": "Add to calendar",
+  "settings.alsoFile": "Also mark the message as read and file it",
   "settings.integrations": "Integrations",
   "settings.jitsi": "Jitsi Meet",
   "settings.jitsiHint":