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)

components/identity/identity-manager-modal.tsx

@@ -145,7 +145,7 @@ export function IdentityManagerModal({ isOpen, onClose }: IdentityManagerModalPr
   if (!isOpen) return null;
 
   return (
-    <div className="fixed inset-0 bg-black/50 flex items-center justify-center z-50 p-4 animate-in fade-in duration-150">
+    <div className="fixed inset-0 bg-black/50 backdrop-blur-[1px] flex items-center justify-center z-50 p-4 animate-in fade-in duration-150">
       <div
         ref={modalRef}
         role="dialog"
@@ -167,7 +167,7 @@ export function IdentityManagerModal({ isOpen, onClose }: IdentityManagerModalPr
           </div>
           <button
             onClick={onClose}
-            className="p-2 rounded-md hover:bg-muted transition-colors text-muted-foreground hover:text-foreground"
+            className="p-1.5 rounded-md hover:bg-muted transition-colors duration-150 text-muted-foreground hover:text-foreground"
           >
             <X className="w-5 h-5" />
           </button>