feat(web): wallpaper system + sticky PageHeader

Wallpaper system with four sources (predefined images, CSS gradients,
custom uploads via mana-media, and theme default). Configurable per-scene
or globally, with overlay controls (blur + opacity) and hover preview.

Adds sticky prop to shared PageHeader component and applies it across
themes, settings, credits, subscription, help, and profile pages.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/shared-theme/src/index.ts

@@ -28,6 +28,16 @@ export type {
 	StartPageConfig,
 	WeekStartDay,
 	GeneralSettings,
+	// Wallpaper Types
+	WallpaperSource,
+	WallpaperSourceNone,
+	WallpaperSourcePredefined,
+	WallpaperSourceGenerated,
+	WallpaperSourceUpload,
+	WallpaperSolid,
+	WallpaperGradient,
+	WallpaperOverlay,
+	WallpaperConfig,
 } from './types';
 
 // User Settings Constants