fix: resolve all 69 ESLint warnings across 40 files

- Remove unused imports (Mail, CalendarIcon, Plus, Circle, cn, Link,
  MessageCircle, User, Tag, FolderUp, useEffect, LogOut, GripVertical,
  Check, X, HoverActionsMode, HoverActionsCorner, asn1js, Convert, etc.)
- Prefix unused variables/params with underscore to satisfy no-unused-vars
- Add missing React hook dependencies (exhaustive-deps)
- Wrap handleNavigateUp in useCallback and selectedGroupMembers in useMemo
- Remove unused eslint-disable directives in jmap/client.ts
- Replace as any with typed casts in filter-store and smime-store tests
- Remove dead code (macOk assignment, unused now variable)

app/admin/themes/page.tsx

@@ -3,7 +3,7 @@
 import { useEffect, useState, useRef } from 'react';
 import { Upload, Trash2, Power, Loader2, Palette, Save, Shield } from 'lucide-react';
 import type { SettingsPolicy } from '@/lib/admin/types';
-import { DEFAULT_POLICY, DEFAULT_THEME_POLICY, DEFAULT_FEATURE_GATES } from '@/lib/admin/types';
+import { DEFAULT_POLICY, DEFAULT_THEME_POLICY } from '@/lib/admin/types';
 
 const BUILTIN_THEME_OPTIONS = [
   { id: 'builtin-nord', name: 'Nord' },