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)

app/[locale]/settings/page.tsx

@@ -66,9 +66,9 @@ export default function SettingsPage() {
                 key={tab.id}
                 onClick={() => setActiveTab(tab.id)}
                 className={cn(
-                  'w-full text-left px-3 py-2 rounded text-sm transition-colors',
+                  'w-full text-left px-3 py-2 rounded-md text-sm transition-colors duration-150',
                   activeTab === tab.id
-                    ? 'bg-accent text-accent-foreground'
+                    ? 'bg-accent text-accent-foreground font-medium'
                     : 'hover:bg-muted text-foreground'
                 )}
               >
@@ -83,10 +83,10 @@ export default function SettingsPage() {
       <div className="flex-1 overflow-y-auto">
         <div className="max-w-3xl mx-auto p-8">
           {/* Page Header */}
-          <div className="mb-8">
-            <div className="flex items-center gap-3 mb-2">
-              <SettingsIcon className="w-8 h-8 text-foreground" />
-              <h1 className="text-3xl font-semibold text-foreground">{t('title')}</h1>
+          <div className="mb-6">
+            <div className="flex items-center gap-2.5 mb-2">
+              <SettingsIcon className="w-6 h-6 text-muted-foreground" />
+              <h1 className="text-2xl font-semibold text-foreground">{t('title')}</h1>
             </div>
           </div>