feat: add privacy mode, thinking levels, PWA support, and i18n

- Add privacy mode to hide IP and user preferences from AI requests
- Add model thinking levels (low/medium/high) for context analysis depth
- Add PWA support with service worker, manifest, and app icons
- Add SettingsPanel for user preferences (theme, background, language)
- Add i18n translations for en/zh/ja/ko/de/fr
- Add Pinia store for centralized settings management
- Update backend to support user preferences and thinking levels
- Update config to use absolute API URLs

public/manifest.webmanifest

@@ -0,0 +1,50 @@
+{
+  "id": "/?source=pwa",
+  "name": "LLM in Text",
+  "short_name": "LLMText",
+  "description": "AI-assisted Markdown editor with real-time completion.",
+  "lang": "zh-CN",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "display_override": [
+    "window-controls-overlay",
+    "standalone",
+    "minimal-ui"
+  ],
+  "orientation": "any",
+  "background_color": "#f8fafc",
+  "theme_color": "#0f172a",
+  "categories": [
+    "productivity",
+    "utilities"
+  ],
+  "icons": [
+    {
+      "src": "/icons/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    }
+  ],
+  "shortcuts": [
+    {
+      "name": "New Draft",
+      "short_name": "Draft",
+      "description": "Open the editor to start writing immediately.",
+      "url": "/"
+    }
+  ]
+}