feat: add Firefox support with cross-browser compatibility

Add manifest.firefox.json for Firefox MV3 (gecko ID, background
scripts instead of service_worker, options_ui). Introduce
browser-polyfill.js shim so all modules use whichever API is
available (browser.* or chrome.*). Add build.sh to target
chrome, firefox, or both.

https://claude.ai/code/session_01Dvgwe7XMoSxnWXkih8p1Cw

src/background/service-worker.js

@@ -2,9 +2,11 @@
  * Silent Send - Background Service Worker
  *
  * Manages badge count, coordinates between popup and content scripts.
+ * Uses `api` alias for cross-browser compatibility (Chrome + Firefox).
  */
 
 import Storage from '../lib/storage.js';
+import api from '../lib/browser-polyfill.js';
 
 // Track substitution counts per tab
 const tabCounts = new Map();
@@ -15,12 +17,12 @@ function updateBadge(tabId) {
   const count = tabCounts.get(tabId) || 0;
   const text = count > 0 ? String(count) : '';
 
-  chrome.action.setBadgeText({ text, tabId });
-  chrome.action.setBadgeBackgroundColor({ color: count > 0 ? '#10b981' : '#6b7280', tabId });
+  api.action.setBadgeText({ text, tabId });
+  api.action.setBadgeBackgroundColor({ color: count > 0 ? '#10b981' : '#6b7280', tabId });
 }
 
 // Reset count when tab navigates
-chrome.tabs.onUpdated.addListener((tabId, changeInfo) => {
+api.tabs.onUpdated.addListener((tabId, changeInfo) => {
   if (changeInfo.status === 'loading') {
     tabCounts.set(tabId, 0);
     updateBadge(tabId);
@@ -28,13 +30,13 @@ chrome.tabs.onUpdated.addListener((tabId, changeInfo) => {
 });
 
 // Cleanup when tab closes
-chrome.tabs.onRemoved.addListener((tabId) => {
+api.tabs.onRemoved.addListener((tabId) => {
   tabCounts.delete(tabId);
 });
 
 // --- Message Handling ---
 
-chrome.runtime.onMessage.addListener((message, sender, sendResponse) => {
+api.runtime.onMessage.addListener((message, sender, sendResponse) => {
   const handler = messageHandlers[message.type];
   if (handler) {
     handler(message, sender, sendResponse);
@@ -87,9 +89,9 @@ const messageHandlers = {
   async 'update:settings'(message) {
     await Storage.saveSettings(message.settings);
     // Broadcast to content scripts
-    const tabs = await chrome.tabs.query({ url: 'https://claude.ai/*' });
+    const tabs = await api.tabs.query({ url: 'https://claude.ai/*' });
     for (const tab of tabs) {
-      chrome.tabs.sendMessage(tab.id, {
+      api.tabs.sendMessage(tab.id, {
         type: 'settings:updated',
         settings: message.settings,
       }).catch(() => {});
@@ -98,6 +100,6 @@ const messageHandlers = {
 };
 
 // --- Set initial badge state ---
-chrome.runtime.onInstalled.addListener(() => {
-  chrome.action.setBadgeBackgroundColor({ color: '#6b7280' });
+api.runtime.onInstalled.addListener(() => {
+  api.action.setBadgeBackgroundColor({ color: '#6b7280' });
 });