Improve favicon contrast and redesign email templates with logo

favicon.svg:
- Light mode: replace near-black fill (#2d3748) with indigo brand color
  (#6366f1 fill, #4f46e5 stroke, #a5b4fc page lines) — distinctive and
  high-contrast without the dark/black appearance
- Dark mode unchanged

email.py:
- Add _EMAIL_LOGO_SVG: inline SVG with white palette for rendering on
  the indigo header (white book, lavender lines, gold sparkle)
- Add _email_html(title, body): shared template wrapper — gray outer
  background, white card with border-radius, indigo header with logo +
  app name, content area, footer

notifications.py:
- Import and use _email_html for all six email functions: security alert,
  password reset, password reset success, invitation, task reminder,
  test email
- Clean up all inline HTML to match the new card layout and spacing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/public/favicon.svg

@@ -1,8 +1,8 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32" width="32" height="32">
   <style>
-    .book { fill: #2d3748; stroke: #4a5568; }
-    .spine { stroke: #4a5568; }
-    .lines { stroke: #a0aec0; }
+    .book { fill: #6366f1; stroke: #4f46e5; }
+    .spine { stroke: #4f46e5; }
+    .lines { stroke: #a5b4fc; }
     @media (prefers-color-scheme: dark) {
       .book { fill: #e2e8f0; stroke: #cbd5e0; }
       .spine { stroke: #cbd5e0; }