feat: add Firefox support (MV3, Firefox 128+)

- Add manifest.firefox.json with gecko-specific settings and background scripts
- Add scripts/build.sh to generate Chrome and Firefox zip packages
- Zero JS changes needed: Firefox 128+ supports chrome.* namespace and world: MAIN
- Closes #10

scripts/build.sh

@@ -0,0 +1,47 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+DIST="$ROOT/dist"
+VERSION=$(grep '"version"' "$ROOT/manifest.json" | head -1 | sed 's/.*: *"\([^"]*\)".*/\1/')
+
+rm -rf "$DIST"
+mkdir -p "$DIST"
+
+SHARED_FILES=(
+  js/background.js
+  js/content.js
+  js/interceptor.js
+  js/patterns.js
+  js/popup.js
+  js/results.js
+  css/popup.css
+  css/results.css
+  icons/icon16.png
+  icons/icon48.png
+  icons/icon128.png
+  popup.html
+  results.html
+)
+
+# --- Chrome build ---
+CHROME_DIR="$DIST/chrome"
+mkdir -p "$CHROME_DIR"/{js,css,icons}
+cp "$ROOT/manifest.json" "$CHROME_DIR/manifest.json"
+for f in "${SHARED_FILES[@]}"; do
+  cp "$ROOT/$f" "$CHROME_DIR/$f"
+done
+(cd "$CHROME_DIR" && zip -r "$DIST/keyfinder-v${VERSION}-chrome.zip" . -x '.*')
+echo "Built: dist/keyfinder-v${VERSION}-chrome.zip"
+
+# --- Firefox build ---
+FF_DIR="$DIST/firefox"
+mkdir -p "$FF_DIR"/{js,css,icons}
+cp "$ROOT/manifest.firefox.json" "$FF_DIR/manifest.json"
+for f in "${SHARED_FILES[@]}"; do
+  cp "$ROOT/$f" "$FF_DIR/$f"
+done
+(cd "$FF_DIR" && zip -r "$DIST/keyfinder-v${VERSION}-firefox.zip" . -x '.*')
+echo "Built: dist/keyfinder-v${VERSION}-firefox.zip"
+
+echo "Done. Upload dist/keyfinder-v${VERSION}-firefox.zip to addons.mozilla.org"