feat(email): alert and weekly-report emails read as designed messages (#5335)

#5335 "Emails Steward sends read as designed messages: an HTML alert email
and weekly report, with a plain-text twin".

- core/mail.py renders each email as multipart/alternative from
  templates/email/: a table-and-inline-style HTML part and a plain-text
  twin carrying the same facts. Colours are the design system's light
  theme, resolved from theme.css; nothing is fetched when a mail opens.
- Alert email: state badge, a headline sentence ("Disk used on web01 is
  94%."), rule, resource, reading, the condition in words, and the time in
  the zone Notifications was saved in with UTC beside it; "View in
  Steward" when Steward's address is set. Subject "[Steward] Firing: <rule>
  — <resource> 94%". Alerts now carry their operator.
- Weekly report: monitors down, active alerts, the week's alert events,
  uptime and busiest Traefik routers as small tables with links. Sent from
  the configured From address; the "Generated … UTC" line and the stale
  repo footer are gone.
- condition_text() states a condition from a rule's parts.

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

tests/integration/test_notification_test_send.py

@@ -111,7 +111,7 @@ def test_test_email_uses_the_form_settings_and_names_the_tls_mode(app, monkeypat
         assert "Sent to a@example.com, b@example.com via mx.example:465 (implicit TLS)." in page
         assert "Rule: Test alert" in page
         assert sent == [{"host": "mx.example", "port": 465, "tls": False, "username": "u",
-                         "password": "pw", "from": "alerts@example.com",
+                         "password": "pw", "from": "alerts@example.com", "tz": "", "base_url": "",
                          "recipients": ["a@example.com", "b@example.com"]}]
 
         resp = await client.post("/settings/notifications/test-email", form={