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/routers/admin.py

@@ -376,7 +376,7 @@ async def test_smtp_settings_endpoint(
 ):
     require_site_admin(current_user)
     try:
-        await send_test_email(db, current_user.email)
+        await send_test_email(db, current_user.email, theme_user=current_user)
     except SmtpNotConfiguredError:
         raise HTTPException(status_code=400, detail="SMTP is not configured yet")
     except Exception as exc: