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/src/lib.rs

@@ -162,6 +162,8 @@ pub struct DownloadProgress {
     pub downloaded_bytes: u64,
     pub total_bytes: u64,
     pub bytes_per_second: u64,
+    /// Unique peers currently streaming at least one chunk for this download.
+    pub active_peer_count: usize,
 }
 
 /// Long-running peer runtime components reported in failure events.