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/thread-conversation-view.tsx

@@ -3,7 +3,7 @@
 import { useState, useEffect, useMemo } from "react";
 import DOMPurify from "dompurify";
 import { Email, ThreadGroup } from "@/lib/jmap/types";
-import { hasRichFormatting, EMAIL_SANITIZE_CONFIG, collapseBlockedImageContainers } from "@/lib/email-sanitization";
+import { EMAIL_SANITIZE_CONFIG, collapseBlockedImageContainers } from "@/lib/email-sanitization";
 import { transformInlineStyles, transformColorForDarkMode, transformBgColorForDarkMode } from "@/lib/color-transform";
 import { useThemeStore } from "@/stores/theme-store";
 import { Avatar } from "@/components/ui/avatar";
@@ -263,9 +263,7 @@ function EmailCard({
 
       if (email.htmlBody?.[0]?.partId && email.bodyValues[email.htmlBody[0].partId]) {
         htmlContent = email.bodyValues[email.htmlBody[0].partId].value;
-
-        // Use safe parsing instead of innerHTML to detect rich formatting
-        useHtmlVersion = hasRichFormatting(htmlContent);
+        useHtmlVersion = !!htmlContent;
       }
 
       if (useHtmlVersion && htmlContent) {