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/email/calendar-invitation-banner.tsx

@@ -286,7 +286,7 @@ export function CalendarInvitationBanner({ email }: CalendarInvitationBannerProp
                 disabled={isProcessing}
                 aria-pressed={currentRsvp === 'accepted'}
                 className={cn(
-                  "flex items-center gap-1 text-sm px-2 py-0.5 rounded transition-colors min-h-[44px] md:min-h-0 disabled:opacity-50",
+                  "flex items-center gap-1 text-sm px-2 py-0.5 rounded-md transition-colors duration-150 min-h-[44px] md:min-h-0 disabled:opacity-50",
                   currentRsvp === 'accepted'
                     ? "bg-green-100 dark:bg-green-900/30 text-green-700 dark:text-green-400 font-medium"
                     : "text-muted-foreground hover:text-foreground hover:bg-muted"
@@ -300,7 +300,7 @@ export function CalendarInvitationBanner({ email }: CalendarInvitationBannerProp
                 disabled={isProcessing}
                 aria-pressed={currentRsvp === 'tentative'}
                 className={cn(
-                  "flex items-center gap-1 text-sm px-2 py-0.5 rounded transition-colors min-h-[44px] md:min-h-0 disabled:opacity-50",
+                  "flex items-center gap-1 text-sm px-2 py-0.5 rounded-md transition-colors duration-150 min-h-[44px] md:min-h-0 disabled:opacity-50",
                   currentRsvp === 'tentative'
                     ? "bg-amber-100 dark:bg-amber-900/30 text-amber-700 dark:text-amber-400 font-medium"
                     : "text-muted-foreground hover:text-foreground hover:bg-muted"
@@ -314,7 +314,7 @@ export function CalendarInvitationBanner({ email }: CalendarInvitationBannerProp
                 disabled={isProcessing}
                 aria-pressed={currentRsvp === 'declined'}
                 className={cn(
-                  "flex items-center gap-1 text-sm px-2 py-0.5 rounded transition-colors min-h-[44px] md:min-h-0 disabled:opacity-50",
+                  "flex items-center gap-1 text-sm px-2 py-0.5 rounded-md transition-colors duration-150 min-h-[44px] md:min-h-0 disabled:opacity-50",
                   currentRsvp === 'declined'
                     ? "bg-red-100 dark:bg-red-900/30 text-red-700 dark:text-red-400 font-medium"
                     : "text-muted-foreground hover:text-foreground hover:bg-muted"
@@ -346,7 +346,7 @@ export function CalendarInvitationBanner({ email }: CalendarInvitationBannerProp
                   }
                 }}
                 disabled={isProcessing}
-                className="flex items-center gap-1 text-sm text-muted-foreground hover:text-foreground hover:bg-muted px-2 py-0.5 rounded transition-colors min-h-[44px] md:min-h-0 disabled:opacity-50"
+                className="flex items-center gap-1 text-sm text-muted-foreground hover:text-foreground hover:bg-muted px-2 py-0.5 rounded-md transition-colors duration-150 min-h-[44px] md:min-h-0 disabled:opacity-50"
               >
                 <CalendarCheck className="w-3.5 h-3.5" />
                 {t('add_to_calendar')}