feat: cross-account "All accounts" views + full group/shared-account support

Add cross-account aggregate mail views and make group/shared (delegated)
accounts first-class in every aggregate view. (The unified mailbox, the "All
Mail" view, and "include group inboxes" already exist on main; this branch adds
the cross-account views and the shared-account correctness work.)

New views (admin-gated + per-user toggle, nested under Unified Mailbox):
- Cross-account "All accounts": All unread / All starred / All mail across every
  connected account, including shared/group folders. Each list labels the source
  folder of every message.

Source reference on aggregated emails (the core of the shared-account work):
- Replace the overloaded `accountId` with two explicit, always-set fields:
  `sourceClientAccountId` (the login the mail is reachable through) and
  `sourceAccountId` (the owning JMAP account). `accountId` stays display-only.
- Resolution is branch-free everywhere: pick the client by sourceClientAccountId,
  pass sourceAccountId as the JMAP accountId (no-op for personal), read the
  owner's mailbox list cached by JMAP id. No capability scan.

Shared/group-account correctness across all aggregate views:
- Route open (click + auto-fetch), thread/conversation open + reply-refresh,
  mark read, star, move, delete (account-scoped trash), archive (owner-routed
  createMailbox / fetchAccountMailboxes), and spam + undo via the source ref.
- Add accountId params to toggleStar / batchMarkAsRead / batchDeleteEmails /
  createMailbox where missing.
- Fix local unread/total counter math for shared folders via emailInMailbox()
  (matches namespaced shared ids and bare own ids).
- Keep the unified/cross virtual selection on background mailbox refresh (no
  jump back to inbox after deleting in All Drafts/Junk).

Junk UX:
- In "All Junk" the spam action becomes "not spam" in the viewer, context menu,
  and list hover icons; undo routes shared mail back to its own inbox.

Admin:
- Policy gates crossUnread/Starred/AllViewEnabled, each noting the matching
  per-user toggle (allMailViewEnabled clarified too).

i18n / docs / tests:
- locales (19): cross-view labels + descriptions and hover not_spam, translated
  in all shipped languages.
- FEATURES.md + README.md document the new views and group-account support.
- Tests for shared-account routing (single + batch + undoSpam), decoration, and
  unified-selection preservation.

app/(main)/admin/_tabs/policy.tsx

@@ -21,7 +21,10 @@ const FEATURE_GATE_LABELS: Partial<Record<keyof FeatureGates, { label: string; d
   folderIconsEnabled: { label: 'Folder Icons', description: 'Allow custom folder icon picker' },
   hoverActionsConfigEnabled: { label: 'Hover Actions Config', description: 'Allow users to customize email hover actions' },
   filesEnabled: { label: 'Files (WebDAV)', description: 'Enable file storage via WebDAV. WARNING: Large uploads can cause Stalwart/RocksDB instability. Not recommended for production.' },
-  allMailViewEnabled: { label: 'All Mail View', description: 'Show a virtual "All Mail" folder that merges messages from across an account’s folders into one list. Users choose which folders are included.' },
+  allMailViewEnabled: { label: 'All Mail View', description: 'Show a virtual "All Mail" folder that merges messages from across an account’s folders into one list. Users choose which folders are included. Requires the per-user toggle in Settings → Appearance.' },
+  crossUnreadViewEnabled: { label: 'All Accounts: Unread', description: 'Allow an "All unread" entry in the All accounts section that lists unread mail across every account (incl. shared folders), spanning all folders except junk, sent, archive, trash and drafts. Requires the matching per-user toggle in Settings → Appearance.' },
+  crossStarredViewEnabled: { label: 'All Accounts: Starred', description: 'Allow an "All starred" entry in the All accounts section that lists flagged/starred mail across every account (incl. shared folders), spanning all folders except junk, sent, archive, trash and drafts. Requires the matching per-user toggle in Settings → Appearance.' },
+  crossAllViewEnabled: { label: 'All Accounts: All Mail', description: 'Allow an "All mail" entry in the All accounts section that lists all mail across every account (incl. shared folders), spanning all folders except junk, sent, archive, trash and drafts. Requires the matching per-user toggle in Settings → Appearance.' },
 };
 
 const RESTRICTABLE_SETTINGS = [