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>

frontend/src/context/AuthContext.tsx

@@ -3,6 +3,7 @@ import * as authApi from '../api/auth'
 import { ApiError, NetworkError } from '../api/client'
 import { clearLastUser, loadLastUser, saveLastUser } from '../lib/lastUser'
 import { unsubscribeFromPush } from '../lib/push'
+import { applyTheme } from '../lib/theme'
 import type { User } from '../types'
 
 interface AuthContextValue {
@@ -22,8 +23,8 @@ export function AuthProvider({ children }: { children: ReactNode }) {
   const [offline, setOffline] = useState(false)
 
   useEffect(() => {
-    document.documentElement.setAttribute('data-theme', user?.theme ?? 'dark')
-  }, [user?.theme])
+    applyTheme(user?.theme ?? null, user?.custom_theme_colors ?? null)
+  }, [user?.theme, user?.custom_theme_colors])
 
   useEffect(() => {
     authApi