feat: P2.1 extended signatures — multiple per identity + TipTap editor

- New stores/signature-store.ts: Zustand persist with CRUD, default/reply
  signature IDs, per-identity signature mapping
- New signature-settings.tsx: list management with add/edit/delete/duplicate
- New signature-editor-modal.tsx: TipTap rich text editor for signatures
- email-composer.tsx: auto-insert signature based on mode (compose/reply)
  + signature selector dropdown in toolbar
- identity-form.tsx: per-identity default/reply signature dropdowns
- settings/page.tsx: Signatures tab in Mail settings group

app/(main)/[locale]/settings/page.tsx

@@ -46,6 +46,7 @@ import { LayoutSettings } from '@/components/settings/layout-settings';
 import { LanguageSettings } from '@/components/settings/language-settings';
 import { ReadingSettings } from '@/components/settings/reading-settings';
 import { ComposingSettings } from '@/components/settings/composing-settings';
+import { SignatureSettings } from '@/components/settings/signature-settings';
 import { ContentSendersSettings } from '@/components/settings/content-senders-settings';
 import { AccountSettings } from '@/components/settings/account-settings';
 import { IdentitySettings } from '@/components/settings/identity-settings';
@@ -98,6 +99,7 @@ type Tab =
   | 'composing'
   | 'downloads'
   | 'identities'
+  | 'signatures'
   | 'vacation'
   | 'filters'
   | 'templates'
@@ -141,6 +143,7 @@ const tabIcons: Record<Tab, LucideIcon> = {
   composing: PenLine,
   downloads: Download,
   identities: UserPen,
+  signatures: PenLine,
   vacation: PalmtreeIcon,
   filters: Filter,
   templates: FileText,
@@ -219,6 +222,7 @@ const tabSearchPaths: Record<Tab, string[]> = {
   ],
   downloads: ['settings.downloads'],
   identities: ['settings.identities'],
+  signatures: ['signatures'],
   vacation: ['settings.vacation'],
   filters: ['settings.filters'],
   templates: ['settings.templates'],
@@ -254,6 +258,7 @@ const tabKeywords: Record<Tab, string> = {
   composing: 'editor signature plain text reply forward draft compose',
   downloads: 'download filename template eml attachment save export',
   identities: 'from address signature email',
+  signatures: 'signature rich text html editor',
   vacation: 'auto reply away out of office holiday responder',
   filters: 'sieve rules block junk forward',
   templates: 'snippet quick reply',
@@ -631,6 +636,7 @@ export default function SettingsPage() {
     { id: 'composing', label: t('tabs.composing'), icon: tabIcons.composing, group: 'mail' },
     { id: 'downloads', label: t('tabs.downloads'), icon: tabIcons.downloads, group: 'mail' },
     { id: 'identities', label: t('tabs.identities'), icon: tabIcons.identities, group: 'mail' },
+    { id: 'signatures', label: t('tabs.signatures'), icon: tabIcons.signatures, group: 'mail' },
     ...(supportsVacation ? [{ id: 'vacation' as Tab, label: t('tabs.vacation'), icon: tabIcons.vacation, group: 'mail' as TabGroup }] : []),
     ...(supportsSieve ? [{ id: 'filters' as Tab, label: t('tabs.filters'), icon: tabIcons.filters, group: 'mail' as TabGroup }] : []),
     ...(isFeatureEnabled('templatesEnabled') ? [{ id: 'templates' as Tab, label: t('tabs.templates'), icon: tabIcons.templates, group: 'mail' as TabGroup }] : []),
@@ -761,6 +767,7 @@ export default function SettingsPage() {
       {effectiveActiveTab === 'composing' && <ComposingSettings />}
       {effectiveActiveTab === 'downloads' && <DownloadsSettings />}
       {effectiveActiveTab === 'identities' && <IdentitySettings />}
+      {effectiveActiveTab === 'signatures' && <SignatureSettings />}
       {effectiveActiveTab === 'vacation' && <VacationSettings />}
       {effectiveActiveTab === 'filters' && <FilterSettings />}
       {effectiveActiveTab === 'templates' && <TemplateSettings />}