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)

stores/ui-store.ts

@@ -4,6 +4,14 @@ import { create } from "zustand";
 
 export type ActiveView = "sidebar" | "list" | "viewer";
 
+// Column width constraints (in pixels)
+const SIDEBAR_MIN = 180;
+const SIDEBAR_MAX = 400;
+const SIDEBAR_DEFAULT = 256;
+const EMAIL_LIST_MIN = 240;
+const EMAIL_LIST_MAX = 600;
+const EMAIL_LIST_DEFAULT = 384;
+
 interface UIState {
   // Mobile view state
   activeView: ActiveView;
@@ -17,12 +25,26 @@ interface UIState {
   isTablet: boolean;
   isDesktop: boolean;
 
+  // Resizable column widths (desktop only)
+  sidebarWidth: number;
+  emailListWidth: number;
+
+  // Sidebar collapsed state (desktop)
+  sidebarCollapsed: boolean;
+
   // Actions
   setActiveView: (view: ActiveView) => void;
   setSidebarOpen: (open: boolean) => void;
   toggleSidebar: () => void;
   setTabletListVisible: (visible: boolean) => void;
   setDeviceType: (isMobile: boolean, isTablet: boolean, isDesktop: boolean) => void;
+  setSidebarWidth: (width: number) => void;
+  setEmailListWidth: (width: number) => void;
+  resetSidebarWidth: () => void;
+  resetEmailListWidth: () => void;
+  persistColumnWidths: () => void;
+  setSidebarCollapsed: (collapsed: boolean) => void;
+  toggleSidebarCollapsed: () => void;
 
   // Navigation helpers
   showEmailList: () => void;
@@ -30,6 +52,8 @@ interface UIState {
   goBack: () => void;
 }
 
+// Column widths are hydrated from localStorage by the page component on mount
+
 export const useUIStore = create<UIState>((set, get) => ({
   // Initial state (SSR-safe defaults)
   activeView: "list",
@@ -38,6 +62,9 @@ export const useUIStore = create<UIState>((set, get) => ({
   isMobile: false,
   isTablet: false,
   isDesktop: true,
+  sidebarWidth: SIDEBAR_DEFAULT,
+  emailListWidth: EMAIL_LIST_DEFAULT,
+  sidebarCollapsed: false,
 
   // Actions
   setActiveView: (view) => set({ activeView: view }),
@@ -51,6 +78,38 @@ export const useUIStore = create<UIState>((set, get) => ({
   setDeviceType: (isMobile, isTablet, isDesktop) =>
     set({ isMobile, isTablet, isDesktop }),
 
+  setSidebarWidth: (width) =>
+    set({ sidebarWidth: Math.min(SIDEBAR_MAX, Math.max(SIDEBAR_MIN, width)) }),
+
+  setEmailListWidth: (width) =>
+    set({ emailListWidth: Math.min(EMAIL_LIST_MAX, Math.max(EMAIL_LIST_MIN, width)) }),
+
+  resetSidebarWidth: () => {
+    set({ sidebarWidth: SIDEBAR_DEFAULT });
+    const { emailListWidth } = get();
+    try {
+      localStorage.setItem("column-widths", JSON.stringify({ sidebarWidth: SIDEBAR_DEFAULT, emailListWidth }));
+    } catch { /* localStorage may be unavailable */ }
+  },
+
+  resetEmailListWidth: () => {
+    set({ emailListWidth: EMAIL_LIST_DEFAULT });
+    const { sidebarWidth } = get();
+    try {
+      localStorage.setItem("column-widths", JSON.stringify({ sidebarWidth, emailListWidth: EMAIL_LIST_DEFAULT }));
+    } catch { /* localStorage may be unavailable */ }
+  },
+
+  persistColumnWidths: () => {
+    const { sidebarWidth, emailListWidth } = get();
+    try {
+      localStorage.setItem("column-widths", JSON.stringify({ sidebarWidth, emailListWidth }));
+    } catch { /* localStorage may be unavailable */ }
+  },
+
+  setSidebarCollapsed: (collapsed) => set({ sidebarCollapsed: collapsed }),
+  toggleSidebarCollapsed: () => set((state) => ({ sidebarCollapsed: !state.sidebarCollapsed })),
+
   // Navigation helpers for mobile
   showEmailList: () => {
     const { isMobile } = get();