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/settings/vacation-settings.tsx

@@ -169,7 +169,7 @@ export function VacationSettings() {
             type="datetime-local"
             value={localFromDate ? utcToLocalDatetime(localFromDate) : ''}
             onChange={(e) => setLocalFromDate(e.target.value ? new Date(e.target.value).toISOString() : '')}
-            className="px-3 py-1.5 text-sm rounded bg-muted border border-border text-foreground focus:outline-none focus:ring-2 focus:ring-primary"
+            className="px-3 py-1.5 text-sm rounded-md bg-muted border border-border text-foreground focus:outline-none focus:ring-2 focus:ring-ring transition-colors duration-150 hover:border-muted-foreground"
           />
         </SettingItem>
         <SettingItem
@@ -180,7 +180,7 @@ export function VacationSettings() {
             type="datetime-local"
             value={localToDate ? utcToLocalDatetime(localToDate) : ''}
             onChange={(e) => setLocalToDate(e.target.value ? new Date(e.target.value).toISOString() : '')}
-            className="px-3 py-1.5 text-sm rounded bg-muted border border-border text-foreground focus:outline-none focus:ring-2 focus:ring-primary"
+            className="px-3 py-1.5 text-sm rounded-md bg-muted border border-border text-foreground focus:outline-none focus:ring-2 focus:ring-ring transition-colors duration-150 hover:border-muted-foreground"
           />
         </SettingItem>
       </SettingsSection>
@@ -195,7 +195,7 @@ export function VacationSettings() {
             value={localSubject}
             onChange={(e) => setLocalSubject(e.target.value)}
             placeholder={t('message.subject_placeholder')}
-            className="w-64 px-3 py-1.5 text-sm rounded bg-muted border border-border text-foreground placeholder:text-muted-foreground focus:outline-none focus:ring-2 focus:ring-primary"
+            className="w-64 px-3 py-1.5 text-sm rounded-md bg-muted border border-border text-foreground placeholder:text-muted-foreground focus:outline-none focus:ring-2 focus:ring-ring transition-colors duration-150 hover:border-muted-foreground"
           />
         </SettingItem>
         <div className="py-3">
@@ -211,7 +211,7 @@ export function VacationSettings() {
             onChange={(e) => setLocalTextBody(e.target.value)}
             placeholder={t('message.body_placeholder')}
             rows={6}
-            className="w-full px-3 py-2 text-sm rounded bg-muted border border-border text-foreground placeholder:text-muted-foreground focus:outline-none focus:ring-2 focus:ring-primary resize-y"
+            className="w-full px-3 py-2 text-sm rounded-md bg-muted border border-border text-foreground placeholder:text-muted-foreground focus:outline-none focus:ring-2 focus:ring-ring transition-colors duration-150 hover:border-muted-foreground resize-y"
           />
         </div>
       </SettingsSection>