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/lib/browser-polyfill.js

@@ -0,0 +1,20 @@
+/**
+ * Minimal browser API compatibility layer.
+ *
+ * Firefox exposes `browser.*` (Promise-based) and polyfills `chrome.*`.
+ * Chrome only has `chrome.*` (callback-based, but storage/runtime are
+ * Promise-based in MV3). This normalizes to whichever is available.
+ */
+
+const api =
+  typeof browser !== 'undefined' && browser.runtime
+    ? browser
+    : typeof chrome !== 'undefined'
+      ? chrome
+      : null;
+
+if (!api) {
+  console.error('[Silent Send] No WebExtension API found');
+}
+
+export default api;