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>

packages/shared-theme/package.json

@@ -0,0 +1,22 @@
+{
+  "name": "@manacore/shared-theme",
+  "version": "0.1.0",
+  "private": true,
+  "type": "module",
+  "main": "./src/index.ts",
+  "types": "./src/index.ts",
+  "exports": {
+    ".": "./src/index.ts",
+    "./store": "./src/store.svelte.ts",
+    "./types": "./src/types.ts",
+    "./constants": "./src/constants.ts",
+    "./utils": "./src/utils.ts"
+  },
+  "peerDependencies": {
+    "svelte": "^5.0.0"
+  },
+  "devDependencies": {
+    "svelte": "^5.0.0",
+    "typescript": "^5.0.0"
+  }
+}