feat: implement unified theme system across all web apps

SUMMARY:
Create a unified theming architecture with two new shared packages
(@manacore/shared-theme and @manacore/shared-theme-ui) that provides
consistent theming across all 4 web applications while allowing
app-specific primary color customization.

NEW PACKAGES:

@manacore/shared-theme:
- Svelte 5 Runes-based theme store factory
- 4 theme variants: Lume (Gold), Nature (Green), Stone (Blue Gray), Ocean (Blue)
- 3 theme modes: Light, Dark, System (auto-detect)
- HSL-based color system with 18 semantic tokens
- localStorage persistence per app
- System preference detection via matchMedia
- Pre-defined configs for all apps (memoro, manacore, manadeck, maerchenzauber)

@manacore/shared-theme-ui:
- ThemeToggle: Light/dark mode toggle button
- ThemeSelector: Visual theme variant selector with color dots
- ThemeModeSelector: Segmented control for light/dark/system

UPDATED PACKAGES:

@manacore/shared-tailwind:
- Converted from HEX to HSL-based CSS variables
- Updated preset.js with hsl(var(--color-*)) syntax
- themes.css now contains all 4 theme variants with light/dark modes

APP MIGRATIONS:

memoro/web:
- Replaced 145 LOC theme store with 25 LOC shared implementation
- Deleted local ThemeSelector.svelte and ThemeToggle.svelte
- Primary color: Gold (47 95% 58%)

manacore/web:
- Replaced 80 LOC theme store with 25 LOC shared implementation
- Deleted local ThemeToggle.svelte
- Primary color: Indigo (239 84% 67%)

manadeck/web:
- Added new theme store using shared package
- Primary color: Indigo (239 84% 67%)

maerchenzauber/web:
- Added new theme store using shared package
- Primary color: Purple (280 60% 55%)

All app layouts updated with theme.initialize() in onMount.

BENEFITS:
- ~225 LOC of app-specific code reduced to ~100 LOC total
- Single source of truth for theme logic
- Consistent theming experience across all apps
- Easy to add new theme variants
- App-specific primary colors preserved

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

manadeck/apps/web/src/lib/stores/theme.ts

@@ -0,0 +1,25 @@
+/**
+ * ManaDeck Theme Store
+ *
+ * Uses the shared theme system with ManaDeck's indigo primary color.
+ */
+import { createThemeStore } from '@manacore/shared-theme';
+
+// Re-export types for convenience
+export type { ThemeMode, ThemeVariant, EffectiveMode } from '@manacore/shared-theme';
+
+/**
+ * ManaDeck theme store instance
+ *
+ * - Default variant: ocean (blue)
+ * - Custom primary: Indigo (#6366f1)
+ * - All 4 theme variants available
+ */
+export const theme = createThemeStore({
+  appId: 'manadeck',
+  defaultVariant: 'ocean',
+  primaryColor: {
+    light: '239 84% 67%', // Indigo #6366f1
+    dark: '239 84% 67%',
+  },
+});

manadeck/apps/web/src/routes/+layout.svelte

@@ -1,8 +1,15 @@
 <script lang="ts">
 	import '../app.css';
 	import favicon from '$lib/assets/favicon.svg';
+	import { theme } from '$lib/stores/theme';
+	import { onMount } from 'svelte';
 
 	let { children } = $props();
+
+	onMount(() => {
+		const cleanup = theme.initialize();
+		return cleanup;
+	});
 </script>
 
 <svelte:head>