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

locales/nl/common.json

@@ -525,7 +525,8 @@
         "label": "Verwijderactie",
         "description": "Wat er gebeurt wanneer je een e-mail verwijdert",
         "trash": "Verplaatsen naar prullenbak",
-        "permanent": "Permanent verwijderen"
+        "permanent": "Permanent verwijderen",
+        "warning": "E-mails worden permanent verwijderd en kunnen niet worden hersteld. Deze actie is onomkeerbaar."
       },
       "show_preview": {
         "label": "Voorbeeldtekst tonen",