fc3g: serve /extension/<filename> with x-xpinstall MIME so Firefox installs in one click

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/frontend.py

@@ -1,13 +1,18 @@
 """Serves the built Vue SPA from frontend/dist/ with history-mode fallback,
-and the on-disk image library + thumbnails from /images.
+the on-disk image library + thumbnails from /images, and the signed
+Firefox extension XPI from frontend/dist/extension/.
 """
 
+import re
 from pathlib import Path
 
-from quart import Blueprint, abort, send_from_directory
+from quart import Blueprint, abort, send_file, send_from_directory
 
 FRONTEND_DIST = Path(__file__).resolve().parent.parent.parent / "frontend" / "dist"
 IMAGES_ROOT = Path("/images")
+XPI_DIR = FRONTEND_DIST / "extension"
+
+_XPI_NAME_RE = re.compile(r"^fabledcurator-[\w.-]+\.xpi$")
 
 frontend_bp = Blueprint("frontend", __name__)
 
@@ -31,6 +36,43 @@ async def serve_image(subpath: str):
     return await send_from_directory(IMAGES_ROOT, subpath)
 
 
+@frontend_bp.route("/extension/<filename>")
+async def serve_extension(filename: str):
+    """Serve the signed FC Firefox extension XPI.
+
+    Path whitelist: filename must match fabledcurator-*.xpi. The special
+    name fabledcurator-latest.xpi serves the most-recently-modified XPI
+    in the directory.
+
+    The application/x-xpinstall MIME tells Firefox to show its native
+    install prompt instead of downloading the file as a blob.
+    """
+    if not _XPI_NAME_RE.fullmatch(filename):
+        abort(404)
+    if not XPI_DIR.is_dir():
+        abort(404)
+    if filename == "fabledcurator-latest.xpi":
+        xpis = sorted(XPI_DIR.glob("fabledcurator-*.xpi"), key=lambda p: p.stat().st_mtime)
+        if not xpis:
+            abort(404)
+        latest = xpis[-1]
+        return await send_file(
+            latest, mimetype="application/x-xpinstall",
+            attachment_filename=latest.name,
+        )
+    target = (XPI_DIR / filename).resolve()
+    try:
+        target.relative_to(XPI_DIR)
+    except ValueError:
+        abort(404)
+    if not target.is_file():
+        abort(404)
+    return await send_file(
+        target, mimetype="application/x-xpinstall",
+        attachment_filename=filename,
+    )
+
+
 @frontend_bp.route("/")
 @frontend_bp.route("/<path:subpath>")
 async def serve_spa(subpath: str = ""):