fix(download): require aria2 for file transfers

Update yt-dlp to 2026.07.04 and refresh the bundled macOS runtime plus engine locks.

Route every non-media download through aria2 by removing the native HTTP fallback path, native GID handling, and the old direct-download harness.

Retry transient aria2 startup/RPC failures before failing, then show the real last error in the download row and Properties modal so Windows failures are diagnosable instead of silent.

Refresh docs and tests around the aria2-only file-download contract, and advance the Firefox extension submodule to its published wording cleanup.

src-tauri/tests/README.md

@@ -1,4 +1,4 @@
-# Headless Download Engine Tests
+# Headless Download Tests
 
 Run the full Rust suite:
 
@@ -7,21 +7,19 @@ cd src-tauri
 cargo test --all-targets
 ```
 
-Run only the async download integration harness with deterministic serial
-performance measurements and visible test output:
+Run the queue-manager harness when changing aria2 scheduling, concurrency, and
+retry behavior:
 
 ```sh
 cd src-tauri
-RUST_BACKTRACE=1 cargo test --test download_engine -- --test-threads=1 --nocapture
+cargo test --test queue_manager -- --nocapture
 ```
 
-The harness binds an ephemeral loopback port and requires no GUI, external
-network access, or bundled media binaries. It validates:
+Run the media metadata smoke test with an explicit URL when changing yt-dlp
+integration:
 
-- aggregation of many streamed HTTP body chunks;
-- pause and ranged resume through `DownloadCoordinator`;
-- cancellation and partial-file cleanup;
-- SHA-256 integrity after resume;
-- retry recovery from transient HTTP failures;
-- terminal error reporting after the retry budget is exhausted;
-- a five-second local transfer performance budget for a 3 MiB fixture.
+```sh
+cd src-tauri
+FIRELINK_LIVE_YOUTUBE_URL='https://www.youtube.com/watch?v=dQw4w9WgXcQ' \
+  cargo test filters_live_youtube_metadata_from_env --lib -- --ignored --nocapture
+```