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>

steward/core/settings.py

@@ -63,6 +63,7 @@ DEFAULTS: dict[str, Any] = {
     "smtp.password": "",
     "smtp.recipients": [],
     "smtp.from": "",
+    "smtp.tz": "",  # the zone of whoever saved Notifications: alert emails give times in it
     "webhook.url": "",
     "webhook.template": _DEFAULT_WEBHOOK_TEMPLATE,
     "ansible.sources": [],
@@ -388,6 +389,8 @@ def to_smtp_cfg(settings: dict[str, Any]) -> dict:
         "password": settings.get("smtp.password", ""),
         "recipients": settings.get("smtp.recipients", []),
         "from": settings.get("smtp.from", ""),
+        "tz": settings.get("smtp.tz", ""),
+        "base_url": settings.get("general.public_base_url", ""),
     }