feat: resizable columns, nav rail overhaul, multi-select & drag-drop, UI polish

Resizable Columns
- Add ResizeHandle component with mouse drag, keyboard (Arrow keys),
  and double-click to reset to default width
- Add sidebarWidth/emailListWidth to ui-store with clamping + persistence
- Wire resize handles between sidebar/email-list panels on desktop

Navigation Rail Overhaul
- Move StorageQuotaCircle, push-status, sign-out from Sidebar to NavigationRail
- Interactive SVG ring with popover breakdown (used/free/total)
- Sidebar collapse state lifted to ui-store
- Show total email count per mailbox alongside unread badge

Email Multi-Selection & Drag-and-Drop
- Ctrl+Click (toggle) and Shift+Click (range) on all list items
- Add selectRangeEmails and lastSelectedEmailId to email-store
- Enable drag-and-drop on thread items and thread headers
- useEmailDrag accepts optional threadEmails for full-thread drag

Email Viewer Layout
- Remove card wrapper for cleaner full-width reading
- Always render HTML body when available
- Adjust skeleton loader to match flat layout

Modal & UI Polish
- Standardise backdrops, close buttons, padding, border-radius, transitions
- Migrate template-string classNames to cn() in settings
- Unify focus-ring token to ring-ring on form controls

i18n
- Add storage_used/free/total keys to all 8 locales

Dev Mock JMAP Server (new, gated by DEV_MOCK_JMAP=true)
- Session, Mailbox/Email/Thread/Identity CRUD, back-references, upload
- GET /download with Content-Disposition, GET /eventsource SSE

Tests (46 new)
- ui-store (13), email-selection (10), resize-handle (9), mock-server (14)

locales/de/common.json

@@ -48,6 +48,9 @@
     "compose": "Verfassen",
     "search_placeholder": "E-Mails durchsuchen...",
     "storage": "Speicher",
+    "storage_used": "Belegt",
+    "storage_free": "Frei",
+    "storage_total": "Gesamt",
     "sign_out": "Abmelden",
     "contacts": "Kontakte",
     "calendar": "Kalender",

locales/en/common.json

@@ -50,6 +50,9 @@
     "search_placeholder": "Search mail...",
     "search_placeholder_hint": "Search mail... (press /)",
     "storage": "Storage",
+    "storage_used": "Used",
+    "storage_free": "Free",
+    "storage_total": "Total",
     "sign_out": "Sign out",
     "contacts": "Contacts",
     "calendar": "Calendar",

locales/es/common.json

@@ -48,6 +48,9 @@
     "compose": "Redactar",
     "search_placeholder": "Buscar correo...",
     "storage": "Almacenamiento",
+    "storage_used": "Usado",
+    "storage_free": "Libre",
+    "storage_total": "Total",
     "sign_out": "Cerrar sesión",
     "contacts": "Contactos",
     "calendar": "Calendario",

locales/fr/common.json

@@ -50,6 +50,9 @@
     "search_placeholder": "Rechercher un email...",
     "search_placeholder_hint": "Rechercher... (appuyez sur /)",
     "storage": "Stockage",
+    "storage_used": "Utilisé",
+    "storage_free": "Libre",
+    "storage_total": "Total",
     "sign_out": "Se déconnecter",
     "contacts": "Contacts",
     "calendar": "Calendrier",

locales/it/common.json

@@ -48,6 +48,9 @@
     "compose": "Scrivi",
     "search_placeholder": "Cerca nella posta...",
     "storage": "Spazio di archiviazione",
+    "storage_used": "Utilizzato",
+    "storage_free": "Libero",
+    "storage_total": "Totale",
     "sign_out": "Esci",
     "contacts": "Contatti",
     "calendar": "Calendario",

locales/ja/common.json

@@ -50,6 +50,9 @@
     "search_placeholder": "メールを検索...",
     "search_placeholder_hint": "メールを検索... (/ を押す)",
     "storage": "ストレージ",
+    "storage_used": "使用中",
+    "storage_free": "空き",
+    "storage_total": "合計",
     "sign_out": "サインアウト",
     "contacts": "連絡先",
     "calendar": "カレンダー",

locales/nl/common.json

@@ -48,6 +48,9 @@
     "compose": "Nieuw bericht",
     "search_placeholder": "Zoeken in e-mail...",
     "storage": "Opslag",
+    "storage_used": "Gebruikt",
+    "storage_free": "Vrij",
+    "storage_total": "Totaal",
     "sign_out": "Afmelden",
     "contacts": "Contacten",
     "calendar": "Agenda",

locales/pt/common.json

@@ -48,6 +48,9 @@
     "compose": "Escrever",
     "search_placeholder": "Buscar e-mails...",
     "storage": "Armazenamento",
+    "storage_used": "Usado",
+    "storage_free": "Livre",
+    "storage_total": "Total",
     "sign_out": "Sair",
     "contacts": "Contatos",
     "calendar": "Calendário",