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/accounts/ssrf.py

@@ -94,6 +94,7 @@ async def guarded_get(url: str) -> str | None:
 class GuardedResponse:
     status: int
     headers: dict[str, str]
+    content: bytes
     text: str
 
 
@@ -131,8 +132,10 @@ async def guarded_request(
             if total > max_bytes:
                 raise SsrfError("response exceeds size cap")
             chunks.append(chunk)
+        body = b"".join(chunks)
         return GuardedResponse(
             status=resp.status_code,
             headers=dict(resp.headers),
-            text=b"".join(chunks).decode("utf-8", "replace"),
+            content=body,
+            text=body.decode("utf-8", "replace"),
         )