Security hardening from the assessment; testing fixes

Security (ultimo-security-report.md):
- Security headers on every response, the app shell included: they lived at
  server level and nginx dropped them in any location with its own header.
  CSP, framing, nosniff, referrer, permissions (docker/nginx-security.conf).
- Plain HTTP forwarded by the reverse proxy is redirected to HTTPS; HSTS on
  HTTPS responses. server_tokens off.
- Cross-site state-changing /api requests refused (login CSRF), WebSocket too.
- Validation errors in the usual error shape, without the request model.
- New-password policy: max 128, common/own-address/repeated refused,
  breached-password check (HIBP k-anonymity, fails open).
- Message HTML: links keep only http(s)/mailto (sanitizer v6); the HTML
  endpoint carries its own no-script CSP.
- Outbound fetches pinned to the checked IP (DNS rebinding); external
  IMAP/SMTP servers re-checked before each connection; CGNAT and mapped
  addresses counted as non-public.

Fixes from testing:
- IDLE loop catches up before idling: mail that came while the worker was
  away no longer waits up to 10 minutes.
- Restore from Spam/Trash/Archive moves only the removed messages, not your
  reply in Sent.
- Narrow desktop list stacks sender/subject/snippet (subject was squeezed
  to nothing at the default width), with hover actions.
- Phone: wide emails scale to fit the screen.
- Singular forms ("1 message"); no zip button for a single attachment;
  app passwords no longer claim "never used" for mail apps.
- ruff format across the backend.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/indexer/sanitize.py

@@ -18,7 +18,8 @@ import nh3
 # Bump when the sanitization policy changes; body_parts.sanitizer_v records it so a
 # render re-sanitizes stale content from the raw blob (SPEC §10.4).
 # v5: this server's own inline-image URLs quoted in a message load from here.
-SANITIZER_VERSION = 5
+# v6: links keep only http(s) and mailto (no data: pages).
+SANITIZER_VERSION = 6
 
 # This server's inline-image URL, however it ended up written: "/api/…",
 # "https://host/api/…", or mangled to "http:///api/…" by an earlier copy.
@@ -202,6 +203,11 @@ def sanitize_html(
             return _sanitize_style(value, allow_remote_images)
         if tag == "a" and attr == "target":
             return "_blank"
+        if attr == "href":
+            # A link goes to the web or to a mail address: data: (a page made
+            # up by the sender) and cid: are kept for inline images only.
+            scheme = value.strip().lower().split(":", 1)[0] if ":" in value else ""
+            return value if scheme in ("", "http", "https", "mailto") else None
         if tag == "img" and attr == "src":
             lowered = value.strip().lower()
             if lowered.startswith("cid:"):