feat: inline search filters, folder icon picker, richer demo data & UI polish

- Replace advanced search panel with inline filter chips and collapsible fields
  (from/to/subject/body inputs, folder dropdown, attachment/flagged/read toggles)
- Add debounced auto-search on filter field changes
- Folder settings: add icon picker for custom per-folder icons, toast feedback,
  reorder sections (folder list first, roles second), empty-state illustration
- Avatar: show sender favicon for company domains, randomuser.me portraits for
  personal emails, custom avatars for demo senders, fallback to initials
- Add /api/favicon proxy route for fetching domain favicons
- Settings store: add senderFavicons toggle and folderIcons persistence
- Richer mock inbox with 11 realistic emails (GitHub, Slack, Stripe, Vercel, etc.)
- Navigation rail & sidebar cleanup, permanent-delete warning i18n (all 8 locales)
- Rename search toggle label to 'More', add body/folder filter translations

components/email/email-list-item.tsx

@@ -102,10 +102,7 @@ export function EmailListItem({ email, selected, onClick, onContextMenu }: Email
       onContextMenu={handleContextMenu}
       style={{ minHeight: 'var(--list-item-height)' }}
     >
-      <div className="flex items-start gap-3 px-4" style={{
-        paddingTop: 'calc((var(--list-item-height) - 40px) / 2)',
-        paddingBottom: 'calc((var(--list-item-height) - 40px) / 2)'
-      }}>
+      <div className="flex items-start gap-3 px-4 py-3">
         {/* Checkbox with smooth animation */}
         <button
           onClick={handleCheckboxClick}