feat(peer): expose active download peer count

The launcher needs design work later for showing how many peers are currently
feeding an active download. Surface that data now on the existing progress
payload so UI state can consume it without a separate event stream or rendering
change.

The peer download tracker now treats each live chunk receive as peer activity
and reports the number of unique peers with in-flight streams. This is a live
transfer count, not the number of peers that advertised the game or received a
plan. Multiple chunks from one peer count once, and the count falls as chunk
streams finish.

Tauri already forwards DownloadGameFilesProgress, so no bridge event was added.
The TypeScript model accepts active_peer_count under download_progress and
preserves it with the same reducer path that keeps bytes and speed while the
backend says the game is still downloading.

Test Plan:
- just fmt
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= just test
- just frontend-test
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= just clippy
- git diff --check
- git diff --cached --check

Refs: none

crates/lanspread-peer/README.md

@@ -75,13 +75,16 @@ When the UI asks to download a game:
    keeps existing data intact and only truncates when we intentionally fall back
    to a full file transfer, which prevents corruption when multiple peers fill
    different regions of the same file.
-5. `version.ini` chunks are buffered in memory and committed last via
+5. `DownloadProgressTracker` samples byte counters, transfer speed, and the
+   number of unique peers that are actively streaming chunks. The Tauri UI sees
+   those values together through the regular download-progress event.
+6. `version.ini` chunks are buffered in memory and committed last via
    `.version.ini.tmp` followed by an atomic rename. Failures are accumulated and
    retried (up to `MAX_RETRY_COUNT`) via `retry_failed_chunks`; failed downloads
    sweep `.version.ini.tmp` and `.version.ini.discarded` without restoring the
    previous sentinel. Cancelled downloads also discard the peer-owned download
    payload while preserving `local/` and install transaction metadata.
-6. After a successful sentinel commit, `PeerEvent::DownloadGameFilesFinished`
+7. After a successful sentinel commit, `PeerEvent::DownloadGameFilesFinished`
    is emitted and the peer auto-runs the install transaction.
 
 `PeerCommand::CancelDownload` cancels the tracked download token for an active