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

build.sh

@@ -0,0 +1,52 @@
+#!/bin/bash
+#
+# Build Silent Send for Chrome or Firefox.
+#
+# Usage:
+#   ./build.sh chrome   → dist/chrome/  (load unpacked)
+#   ./build.sh firefox  → dist/firefox/ (load as temporary add-on)
+#   ./build.sh both     → builds both
+#
+
+set -e
+
+TARGET="${1:-both}"
+
+build() {
+  local browser="$1"
+  local out="dist/$browser"
+
+  echo "Building for $browser..."
+  rm -rf "$out"
+  mkdir -p "$out"
+
+  # Copy all source files
+  cp -r src icons "$out/"
+
+  # Copy the correct manifest
+  if [ "$browser" = "firefox" ]; then
+    cp manifest.firefox.json "$out/manifest.json"
+  else
+    cp manifest.json "$out/manifest.json"
+  fi
+
+  # Copy other root files
+  cp README.md "$out/" 2>/dev/null || true
+
+  echo "  → $out/ ready"
+}
+
+case "$TARGET" in
+  chrome)  build chrome ;;
+  firefox) build firefox ;;
+  both)    build chrome; build firefox ;;
+  *)
+    echo "Usage: $0 {chrome|firefox|both}"
+    exit 1
+    ;;
+esac
+
+echo ""
+echo "Done! Load the extension:"
+echo "  Chrome:  chrome://extensions → Load unpacked → dist/chrome/"
+echo "  Firefox: about:debugging → This Firefox → Load Temporary Add-on → dist/firefox/manifest.json"