feat: add cross-browser builds and refresh project documentation

- add Chrome/Chromium Manifest V3 build with PAC proxy routing
- keep Firefox Manifest V2 and Android compatibility
- add Chromium API adapter and service worker
- add Chrome build script and Makefile target
- update README and add AGENT.md
- document installation, signing, permissions, themes, and platform
  limitations

background-chrome.js

@@ -0,0 +1,102 @@
+// Chromium implementation. Chrome's proxy API uses a PAC script rather than
+// Firefox's per-request proxy.onRequest listener.
+let config = configManager.getConfig();
+
+async function init() {
+  await configManager.load();
+  config = configManager.getConfig();
+  await applyProxyConfig();
+  configManager.addListener(() => {
+    config = configManager.getConfig();
+    applyProxyConfig().catch(error => console.error('Failed to apply proxy config:', error));
+  });
+}
+
+function escapePacString(value) {
+  return JSON.stringify(String(value));
+}
+
+function buildPacScript() {
+  const rules = {};
+  Object.entries(config.domains || {}).forEach(([domain, entry]) => {
+    if (!entry || !entry.enabled || !entry.host || !entry.port) return;
+    const scheme = entry.type === 'socks' ? 'SOCKS5' : 'PROXY';
+    rules[domain] = `${scheme} ${entry.host}:${parseInt(entry.port, 10)}`;
+  });
+
+  return `function FindProxyForURL(url, host) {
+    var rules = ${JSON.stringify(rules)};
+    return Object.prototype.hasOwnProperty.call(rules, host) ? rules[host] : "DIRECT";
+  }`;
+}
+
+async function applyProxyConfig() {
+  const hasRules = Object.values(config.domains || {}).some(entry =>
+    entry && entry.enabled && entry.host && entry.port
+  );
+
+  return chrome.proxy.settings.set({
+    value: hasRules
+      ? { mode: 'pac_script', pacScript: { data: buildPacScript(), mandatory: true } }
+      : { mode: 'direct' },
+    scope: 'regular'
+  });
+}
+
+function getDomain(url) {
+  try { return new URL(url).hostname.replace(/^www\./, ''); } catch (_) { return null; }
+}
+
+function sendConfigResponse(sendResponse, action) {
+  if (action === 'getConfig') sendResponse({ config: configManager.getConfig() });
+  else if (action === 'getProfiles') sendResponse({ profiles: configManager.getProfiles() });
+  else return false;
+  return true;
+}
+
+chrome.webRequest.onAuthRequired.addListener(
+  (details, sendResponse) => {
+    if (!details.isProxy) return sendResponse({});
+    const entry = config.domains && config.domains[getDomain(details.url)];
+    if (!entry || !entry.enabled || !entry.username || !entry.password) return sendResponse({});
+    sendResponse({ authCredentials: { username: entry.username, password: entry.password } });
+  },
+  { urls: ['<all_urls>'] },
+  ['blocking']
+);
+
+chrome.runtime.onMessage.addListener((message, sender, sendResponse) => {
+  const direct = sendConfigResponse(sendResponse, message.action);
+  if (direct) return true;
+
+  const asyncActions = {
+    updateDomain: () => configManager.updateDomain(message.domain, message.domainConfig),
+    removeDomain: () => configManager.removeDomain(message.domain),
+    updateProfile: () => configManager.updateProfile(message.profileName, message.profileConfig),
+    removeProfile: () => configManager.removeProfile(message.profileName),
+    updateSettings: () => configManager.updateSettings(message.settings || {}),
+    reloadConfig: () => configManager.reload(),
+    clearAllDomains: () => { configManager.config.domains = {}; return configManager.save(); },
+    clearAllProfiles: () => { configManager.config.profiles = {}; return configManager.save(); },
+    clearEverything: () => { configManager.config.domains = {}; configManager.config.profiles = {}; return configManager.save(); }
+  };
+
+  if (message.action === 'getProfile') {
+    sendResponse({ profile: configManager.getProfileConfig(message.profileName) });
+    return false;
+  }
+  if (message.action === 'toggleProxy') {
+    const entry = configManager.getDomainConfig(message.domain);
+    if (!entry) { sendResponse({ success: false, error: 'Domain not configured' }); return false; }
+    entry.enabled = message.enabled;
+    asyncActions.updateDomain = () => configManager.updateDomain(message.domain, entry);
+  }
+  const operation = asyncActions[message.action];
+  if (!operation) return false;
+  operation().then(() => sendResponse({ success: true })).catch(error =>
+    sendResponse({ success: false, error: error.message })
+  );
+  return true;
+});
+
+init().catch(error => console.error('Failed to initialize Chrome proxy support:', error));