fix(download): emit failure events on early-returns and update UI transition

Address backend early-return paths that were silently exiting without emitting a
terminal event to the UI, and align the UI transition to "Downloading" with the
actual start of the chunk transfer.

- Added `DownloadGameFilesFailed` event emissions to `handlers.rs` in the
  unhandled early-return branches (when resolved file descriptions are empty or
  when no trusted peers are found without a local copy). This prevents the UI
  from getting stuck in a checking state.
- Updated the frontend `'game-download-pre'` listener to keep the status in
  `CheckingPeers` during peer majority size validation, and let the UI switch
  to `Downloading` only upon `'game-download-begin'`.
- Added clarifying comments explaining the safety and semantic roles of both
  listeners.

Test Plan:
- Run all unit tests to ensure no regressions: `just test`
- Compile and build the Tauri project: `just build`

crates/lanspread-peer/src/handlers.rs

@@ -251,6 +251,9 @@ pub async fn handle_download_game_files_command(
         log::error!(
             "No validated file descriptions available to download game {id}; request metadata first"
         );
+        if let Err(send_err) = tx_notify_ui.send(PeerEvent::DownloadGameFilesFailed { id }) {
+            log::error!("Failed to send DownloadGameFilesFailed event: {send_err}");
+        }
         return;
     }
 
@@ -277,6 +280,11 @@ pub async fn handle_download_game_files_command(
             }
         } else {
             log::error!("No trusted peers available after majority validation for game {id}");
+            if let Err(send_err) =
+                tx_notify_ui.send(PeerEvent::DownloadGameFilesFailed { id: id.clone() })
+            {
+                log::error!("Failed to send DownloadGameFilesFailed event: {send_err}");
+            }
         }
         return;
     }