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/ui/confirm-dialog.tsx

@@ -56,7 +56,7 @@ export function ConfirmDialog({
   const resolvedCancelText = cancelText || t("cancel");
 
   return (
-    <div className="fixed inset-0 bg-black/50 backdrop-blur-[2px] flex items-center justify-center z-[60] 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-[60] p-4 animate-in fade-in duration-150">
       <div
         ref={dialogRef}
         role="alertdialog"

components/ui/context-menu.tsx

@@ -74,8 +74,8 @@ export function ContextMenuItem({
       role="menuitem"
       disabled={disabled}
       className={cn(
-        "w-full px-3 py-2 text-sm text-left flex items-center gap-2",
-        "transition-colors duration-100",
+        "w-full px-3 py-1.5 text-sm text-left flex items-center gap-2",
+        "transition-colors duration-150",
         "focus:outline-none focus:bg-muted",
         disabled && "opacity-50 cursor-not-allowed",
         !disabled && "hover:bg-muted cursor-pointer",
@@ -157,8 +157,8 @@ export function ContextMenuSubMenu({
     >
       <div
         className={cn(
-          "w-full px-3 py-2 text-sm flex items-center gap-2",
-          "transition-colors duration-100 cursor-pointer",
+          "w-full px-3 py-1.5 text-sm flex items-center gap-2",
+          "transition-colors duration-150 cursor-pointer",
           "hover:bg-muted"
         )}
         role="menuitem"

components/ui/welcome-banner.tsx

@@ -69,10 +69,10 @@ export function WelcomeBanner() {
           </div>
           <button
             onClick={dismiss}
-            className="flex-shrink-0 p-1 rounded hover:bg-muted transition-colors"
+            className="flex-shrink-0 p-1.5 rounded-md hover:bg-muted transition-colors duration-150 text-muted-foreground hover:text-foreground"
             aria-label={t("dismiss")}
           >
-            <X className="w-4 h-4 text-muted-foreground" />
+            <X className="w-4 h-4" />
           </button>
         </div>
         <div className="mt-3 flex justify-end">