Style outgoing emails instead of plain text (#68)

Every email went through one shared plain-text-only path. Redesigned
send_email/send_test_email around structured paragraphs + an optional
CTA button instead of one pre-formatted string, and render both a
proper styled HTML card (table-based, inline styles -- email clients
strip <style> blocks and don't support CSS variables) and a clean
plain-text fallback from the same input, sent as multipart/alternative.

The HTML is themed per recipient: an email to an existing user renders
in their own selected theme (dark/light/midnight/sunset, or their saved
custom palette), resolved server-side from User.theme/
active_custom_theme_id. Site invites have no account yet to read a
theme from, so they use the default DarkSingularity palette. All five
existing email triggers (site invite, room-added, password reset,
#66's DM notification, admin test email) updated to the new call shape.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/tests/test_rooms.py

@@ -450,8 +450,8 @@ async def test_change_member_role_owner_only(client, db_session):
 def _fake_send_email(monkeypatch):
     calls = []
 
-    async def fake(db, to, subject, body):
-        calls.append({"to": to, "subject": subject, "body": body})
+    async def fake(db, to, subject, paragraphs, **kwargs):
+        calls.append({"to": to, "subject": subject, "paragraphs": paragraphs, **kwargs})
 
     monkeypatch.setattr("app.services.room_service.send_email", fake)
     return calls