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>

docker/nginx-app.conf

@@ -20,11 +20,10 @@ gzip_min_length 1024;
 gzip_types text/plain text/css application/javascript application/json
            image/svg+xml application/manifest+json;
 
-# Clickjacking / sniffing basics. The app's own CSP for mail HTML is applied
-# server-side in app/indexer/sanitize.py; this is only the shell document.
-add_header X-Content-Type-Options nosniff always;
-add_header X-Frame-Options SAMEORIGIN always;
-add_header Referrer-Policy no-referrer always;
+# Security headers (CSP, framing, HSTS, …) for everything this listener serves.
+# Every location below that adds a header of its own includes the file again —
+# see the note at the top of it.
+include /etc/nginx/webmail-security.conf;
 
 # ---- API ----------------------------------------------------------------
 location /api/ {
@@ -72,12 +71,14 @@ location ~ ^/\.well-known/(caldav|carddav)$ {
 location /assets/ {
     expires 1y;
     add_header Cache-Control "public, immutable";
+    include /etc/nginx/webmail-security.conf;
     try_files $uri =404;
 }
 
 location /fonts/ {
     expires 30d;
     add_header Cache-Control "public";
+    include /etc/nginx/webmail-security.conf;
     try_files $uri =404;
 }
 
@@ -85,6 +86,7 @@ location = /healthz {
     access_log off;
     return 200 "ok\n";
     add_header Content-Type text/plain;
+    include /etc/nginx/webmail-security.conf;
 }
 
 # The SPA fallback lives in each server block, not here: the two listeners serve
@@ -95,12 +97,14 @@ location = /healthz {
 # because the public listener 404s /admin.html before it can be served.
 location = /index.html {
     add_header Cache-Control "no-store, must-revalidate";
+    include /etc/nginx/webmail-security.conf;
 }
 
 # The offline-access service worker (frontend/public/sw.js): a fixed URL the
 # browser re-checks for a new version, so it must not be cached either.
 location = /sw.js {
     add_header Cache-Control "no-cache";
+    include /etc/nginx/webmail-security.conf;
     try_files $uri =404;
 }
 
@@ -110,5 +114,6 @@ location = /manifest.webmanifest {
     default_type application/manifest+json;
     types { }
     add_header Cache-Control "no-cache";
+    include /etc/nginx/webmail-security.conf;
     try_files $uri =404;
 }