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/app/services/message_events.py

@@ -215,7 +215,10 @@ async def _maybe_email_dm_notification(
         db,
         recipient.email,
         f"New message from {sender.username}",
-        f"{body_line}\n\nView it here:\n{link}",
+        [body_line],
+        cta_label="Open conversation",
+        cta_url=link,
+        theme_user=recipient,
     )