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`
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