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)

hooks/use-email-drag.ts

@@ -8,6 +8,7 @@ import { useDragDropContext } from "@/contexts/drag-drop-context";
 interface UseEmailDragOptions {
   email: Email;
   sourceMailboxId: string;
+  threadEmails?: Email[];
 }
 
 interface UseEmailDragReturn {
@@ -42,18 +43,19 @@ function createDragPreview(count: number): HTMLElement {
   return preview;
 }
 
-export function useEmailDrag({ email, sourceMailboxId }: UseEmailDragOptions): UseEmailDragReturn {
+export function useEmailDrag({ email, sourceMailboxId, threadEmails }: UseEmailDragOptions): UseEmailDragReturn {
   const { selectedEmailIds, emails } = useEmailStore();
   const { startDrag, endDrag, isDragging, draggedEmails } = useDragDropContext();
 
   const handleDragStart = useCallback((e: DragEvent<HTMLDivElement>) => {
     // Determine which emails to drag:
     // - If current email is selected, drag all selected
+    // - If threadEmails provided (thread header), drag all thread emails
     // - Otherwise, drag only this email
     const isSelected = selectedEmailIds.has(email.id);
     const emailsToDrag = isSelected
       ? emails.filter(em => selectedEmailIds.has(em.id))
-      : [email];
+      : threadEmails || [email];
 
     // Set data transfer
     e.dataTransfer.effectAllowed = "move";
@@ -76,7 +78,7 @@ export function useEmailDrag({ email, sourceMailboxId }: UseEmailDragOptions): U
     });
 
     startDrag(emailsToDrag, sourceMailboxId);
-  }, [email, selectedEmailIds, emails, sourceMailboxId, startDrag]);
+  }, [email, selectedEmailIds, emails, sourceMailboxId, startDrag, threadEmails]);
 
   const handleDragEnd = useCallback(() => {
     endDrag();