Add custom theme colors, full per-token control (#30)

Adds a 5th "Custom" option to the theme swatch grid alongside the
existing 4 presets, opening a picker for all ~12 CSS custom properties
(backgrounds, borders, text, three accent tiers, highlight, danger) plus
a light/dark toggle for native control rendering.

Persisted as a new users.custom_theme_colors JSONB column, validated
server-side against exactly what a native <input type="color"> can ever
produce. Colors survive switching to a preset and back, since there's no
reason picking a preset for a moment should force redoing every color
pick. Applied at runtime as inline custom properties on :root (presets
stay static CSS) via a shared applyTheme() helper, which is also
responsible for clearing those inline overrides when switching away --
otherwise they'd silently keep winning over whatever preset's own
stylesheet values should apply next.

Live preview on every color change; persists only on explicit "Save
colors" (not per keystroke, since a color input fires continuously while
dragging), and closing the modal without saving reverts the preview back
to whatever's actually persisted.

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

backend/app/routers/auth.py

@@ -85,6 +85,8 @@ async def update_profile(
         current_user.display_name = display_name or None
     if "theme" in updates:
         current_user.theme = updates["theme"]
+    if "custom_theme_colors" in updates:
+        current_user.custom_theme_colors = updates["custom_theme_colors"]
     if "appear_offline" in updates:
         current_user.appear_offline = updates["appear_offline"]
     await db.commit()