Sender avatars, PDF password preview, contacts autosave, account-view + Trash/Spam auto-purge, UI polish

Sender avatars (privacy-safe, server-fetched, cached):
- New GET /api/avatar resolves Gravatar (d=404) then the sender domain's favicon
  under the SSRF guard, Redis-cached (hit+miss); gated by SENDER_AVATARS_ENABLED.
  GuardedResponse gains raw bytes. Shown in the mail list, reading pane, People
  panel, and Contacts (overlay that falls back to initials on a miss).

PDF preview:
- Password-protected PDFs now prompt in-place and retry (pdf.js PasswordException),
  with a clear error for other load failures instead of a blank overlay. The
  password is remembered per attachment until the thread is left; clicking the
  field no longer dismisses the overlay.

Contacts:
- Autosave reply recipients: message.sent now carries to/cc (name+email); the
  contacts plugin subscribes and adds any non-contact recipients to the Autosaved
  group. Contact avatars show the gravatar/favicon via /api/avatar.

Mail views / retention:
- Clicking an account shows all its mail except Trash/Spam (was inbox-only).
- Daily cron autopurge_trash_spam permanently deletes Trash/Spam older than
  TRASH_RETENTION_DAYS (default 30; 0 disables) — server EXPUNGE + local rows.

UI polish:
- Ultimo signet logo in the Mail/Calendar/Contacts sidebar header and collapsed
  rail; consistent 11px top margin and vertically-aligned header row across states
  (fixed-size flex icon buttons); SVG favicon on the browser tab.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U72FZiF1W1pb8XxyWBTREA

backend/app/config.py

@@ -62,6 +62,11 @@ class Settings(BaseSettings):
     # Gates the "Sign in with Google instead" OAuth fallback in the wizard (SPEC
     # §6.2.1). Off until an operator configures a verified Cloud project.
     google_oauth_enabled: bool = Field(default=False, alias="GOOGLE_OAUTH_ENABLED")
+    # Server-side sender avatars (Gravatar + domain favicon). Fetched by the server
+    # (never the browser) so senders can't see the user's IP or read activity.
+    sender_avatars_enabled: bool = Field(default=True, alias="SENDER_AVATARS_ENABLED")
+    # Auto-purge Trash/Spam messages older than this many days (0 disables).
+    trash_retention_days: int = Field(default=30, alias="TRASH_RETENTION_DAYS")
 
 
 @lru_cache(maxsize=1)