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/password_service.py

@@ -46,10 +46,13 @@ async def request_password_reset(db: AsyncSession, email: str, base_url: str) ->
         db,
         email,
         "Reset your DS Chat password",
-        f"Someone requested a password reset for this account.\n\n"
-        f"Reset it here:\n{reset_link}\n\n"
-        f"This link expires in 15 minutes. If you didn't request this, "
-        f"you can ignore this email.",
+        [
+            "Someone requested a password reset for this account.",
+            "This link expires in 15 minutes. If you didn't request this, you can ignore this email.",
+        ],
+        cta_label="Reset password",
+        cta_url=reset_link,
+        theme_user=user,
     )