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>

package.json

@@ -15,7 +15,10 @@
     "maerchenzauber:dev": "turbo run dev --filter=maerchenzauber...",
     "manacore:dev": "turbo run dev --filter=manacore...",
     "manadeck:dev": "turbo run dev --filter=manadeck...",
-    "memoro:dev": "turbo run dev --filter=memoro..."
+    "memoro:dev": "turbo run dev --filter=memoro...",
+    "dev:web": "turbo run dev --filter=@storyteller/web --filter=manacore-web --filter=web --filter=memoro-web",
+    "dev:landing": "turbo run dev --filter=@storyteller/landing --filter=manacore-landing --filter=landing --filter=memoro-landing",
+    "dev:mobile": "turbo run dev --filter=@storyteller/mobile --filter=manacore --filter=manadeck --filter=memoro"
   },
   "devDependencies": {
     "prettier": "^3.3.3",