feat(unified-mailbox): account-bounded Unified Mailbox with opt-in cross-account

Rework the sidebar "All accounts" section into a "Unified Mailbox" that, by
default, stays within the active login account and its shared/group folders.
Merging across multiple logged-in accounts becomes an opt-in sub-option instead
of the default, and the standalone per-account "All Mail" virtual folder is
folded into the unified All mail / Unread / Starred entries (its folder selection
now narrows those lists).

Scope:
- lib/unified-mailbox.ts: UnifiedAccountClient.crossIncludedMailboxIds; the cross
  views honor the per-account folder selection (union across accounts = the sum
  of each account's selection), falling back to inbox+custom when unset.
- stores/email-store.ts: buildUnifiedAccountClients gains scopeToClientAccountId
  (the account boundary) and populates crossIncludedMailboxIds from
  allMailFolderIds; remove the standalone __all_mail__ fetch/search/load-more
  branches.
- page.tsx: scope to the active account unless cross-account is active (per-user
  opt-in AND admin gate); the per-role unified mailboxes obey the same scope.

Folding:
- Drop ALL_MAIL_MAILBOX_ID (lib/jmap/types.ts); thread-list source-folder column
  now keys on isUnifiedView only; settings folder picker moves under the unified
  group and shows once any unified entry is enabled.

Config:
- User: new unifiedCrossAccount (default false); includeGroupInUnified default
  flips to true; enableAllMailView retired; the three cross-view toggles now gate
  the unified Unread/Starred/All mail entries.
- Admin: new unifiedCrossAccountEnabled gate, default FALSE (cross-account is an
  admin opt-in; when off the per-user toggle is hidden and the scope is forced
  account-bounded at runtime). allMailViewEnabled deprecated and normalized
  forward into crossAllViewEnabled on policy load; cross-view gate labels reworded
  to "Unified Mailbox: ...".

Header: the sidebar section shows "All accounts" when cross-account is active
(opt-in AND admin gate AND >1 connected account), else "Unified Mailbox".

Migration:
- Settings persist v5 -> v6 (exported migrateSettings) - cross-active users keep
  cross-account; All-Mail-only users get the account-bounded unified All mail
  entry with folder ids preserved; includeGroupInUnified enabled for every
  migrated config; fresh installs are account-bounded.
- Admin policy: one-shot, marker-guarded migratePolicyUnifiedMailbox (run before
  configManager.load) enables unifiedCrossAccountEnabled when a cross view was
  active, so existing cross-account installs keep the behaviour despite the
  default-false gate. Skipped on read-only config dirs.

Locales: sidebar all_accounts (original label) + unified_mailbox (translated, per
locale) keys; dead standalone all_mail strings removed across all 20 locales.

Docs: FEATURES.md updated to the account-bounded model, the cross-account gate,
and the folder-narrowed aggregate entries.

Verification: tsc clean, eslint clean, full vitest suite green (incl. translations
completeness, cross-view/migration coverage, and the admin policy migration test).

FEATURES.md

@@ -4,9 +4,8 @@
 
 - Read, compose, reply, reply-all, and forward with a Tiptap rich text editor (inline images, drag-and-drop embedding, tables)
 - Gmail-style threading with inline expansion and an optional conversation toggle
-- Unified mailbox view across all connected accounts – combined Inbox, Sent, Drafts, Junk, Archive, and Trash, with group/shared accounts optionally merged in
-- Cross-account "All accounts" views – All unread, All starred, and All mail spanning every account (including shared/group folders); each aggregate list labels the source folder of every message
-- "All Mail" view that merges an account's folders (with a configurable folder selection) into a single list
+- Unified Mailbox – combined Inbox, Sent, Drafts, Junk, Archive, and Trash, scoped by default to the active account and its shared/group folders, with an optional admin-gated cross-account mode that spans every connected account
+- Aggregated All mail / Unread / Starred entries in the Unified Mailbox – scoped by the same account boundary (or all accounts in cross-account mode) and narrowed by a per-account folder selection; each list labels the source folder of every message
 - Three selectable mail layouts: split (three-pane), focused list, and reading pane at bottom
 - Draft auto-save with identity preservation, persisted HTML body, and proper `In-Reply-To` / `References` headers on replies
 - Attachment upload, download, drag-out to local file system, and inline preview – images, inline PDF on desktop and mobile, composer attachments (click to open), and `.eml` (`message/rfc822`) attachments rendered like an email; image thumbnails and forgotten-attachment warning
@@ -117,7 +116,7 @@ Automatic browser detection with persistent preference. Configurable locale URL
 - Configurable signature position (above or below quoted text)
 - Sub-addressing (`user+tag@domain.com`) with configurable delimiter and contextual tag suggestions
 - Shared folders across accounts
-- Shared / group (delegated) accounts: their folders appear alongside your own and can be merged into the unified and "All accounts" views ("Include group inboxes"); their messages are fully actionable there – open, mark read, spam / not-spam, move, delete, and archive – with folder unread counts kept in sync
+- Shared / group (delegated) accounts: their folders appear alongside your own and can be merged into the Unified Mailbox ("Include group inboxes"); their messages are fully actionable there – open, mark read, spam / not-spam, move, delete, and archive – with folder unread counts kept in sync
 - Multiple JMAP servers per deployment with optional auto-pick by email domain
 - Optional custom JMAP endpoints on the login form (`ALLOW_CUSTOM_JMAP_ENDPOINT`)
 
@@ -125,7 +124,7 @@ Automatic browser detection with persistent preference. Configurable locale URL
 
 - Web setup wizard for first launch – guides through JMAP server(s), OAuth/OIDC, session secret, logging, branding (with file upload), and admin password; persists to the admin config dir, no `.env.local` editing required
 - Stalwart admin dashboard with dedicated policy sections, collapsed into a single tabbed page
-- Admin policy gates for the aggregate mail views – enable or disable the "All Mail" and the cross-account "All unread / starred / all" entries org-wide; each gated view still respects the user's own toggle
+- Admin policy gates for the Unified Mailbox – enable or disable the All mail / Unread / Starred entries org-wide, plus a cross-account capability gate (off by default; auto-enabled on upgrade for instances that already used the cross-account views); each gated view still respects the user's own toggle
 - Split admin storage: `ADMIN_CONFIG_DIR` (operator-authored, mountable read-only after setup) and `ADMIN_STATE_DIR` (runtime audit log and login timestamps)
 - File-based secrets for JSON config: `passwordHashFile` (admin password), `sessionSecretFile`, and `oauthClientSecretFile` for Docker/Kubernetes secret mounts
 - Admin toggle for search-engine indexing (`robots.txt` / `noindex`)