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/sw.js

@@ -0,0 +1,79 @@
+const CACHE_NAME = 'llm-in-text-v1';
+const APP_SHELL_ASSETS = [
+  '/',
+  '/index.html',
+  '/manifest.webmanifest',
+  '/icons/icon-180.png',
+  '/icons/icon-192.png',
+  '/icons/icon-512.png'
+];
+
+self.addEventListener('install', (event) => {
+  event.waitUntil(
+    caches.open(CACHE_NAME).then((cache) => cache.addAll(APP_SHELL_ASSETS))
+  );
+  self.skipWaiting();
+});
+
+self.addEventListener('activate', (event) => {
+  event.waitUntil(
+    caches.keys().then((keys) =>
+      Promise.all(
+        keys
+          .filter((key) => key !== CACHE_NAME)
+          .map((key) => caches.delete(key))
+      )
+    )
+  );
+  self.clients.claim();
+});
+
+async function staleWhileRevalidate(request) {
+  const cache = await caches.open(CACHE_NAME);
+  const cached = await cache.match(request);
+  const networkPromise = fetch(request)
+    .then((response) => {
+      if (response.ok) {
+        cache.put(request, response.clone());
+      }
+      return response;
+    })
+    .catch(() => cached);
+
+  return cached || networkPromise;
+}
+
+self.addEventListener('fetch', (event) => {
+  const { request } = event;
+
+  if (request.method !== 'GET') return;
+
+  const url = new URL(request.url);
+  if (url.origin !== self.location.origin) return;
+
+  if (url.pathname.startsWith('/v1/')) return;
+
+  if (request.mode === 'navigate') {
+    event.respondWith(
+      fetch(request)
+        .then((response) => {
+          if (response.ok) {
+            const copy = response.clone();
+            caches.open(CACHE_NAME).then((cache) => cache.put(request, copy));
+          }
+          return response;
+        })
+        .catch(async () => {
+          const cachedPage = await caches.match(request);
+          if (cachedPage) return cachedPage;
+          return caches.match('/index.html');
+        })
+    );
+    return;
+  }
+
+  const cacheableDestinations = ['script', 'style', 'font', 'image', 'worker'];
+  if (cacheableDestinations.includes(request.destination)) {
+    event.respondWith(staleWhileRevalidate(request));
+  }
+});