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/thread-list-item.tsx

@@ -96,12 +96,7 @@ const SingleEmailItem = React.forwardRef<HTMLDivElement, SingleEmailItemProps>(
         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="w-6 flex-shrink-0" />
-
+        <div className="flex items-start gap-3 px-3 py-3">
           {isUnread && (
             <div className="absolute left-1 top-1/2 -translate-y-1/2">
               <Circle className="w-2 h-2 fill-blue-600 text-blue-600 dark:fill-blue-400 dark:text-blue-400" />
@@ -275,10 +270,7 @@ export const ThreadListItem = React.forwardRef<HTMLDivElement, ThreadListItemPro
           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-3 py-3">
             {!isMobile && (
               <button
                 data-expand-toggle