fix(peer): delete partial files when a download is cancelled

Cancelling an in-flight download via `PeerCommand::CancelDownload` previously
torn down the network transfer and cleared `active_downloads`, but left the
partial `.eti` archive(s) sitting in the game root forever. The next library
scan still picked up the half-written files as a "downloaded" game, and the
only escape was the `Remove files` action. This is the symmetric fix to
`62ceb06 feat(peer): remove downloaded game files safely`: the cancel path
must clean up after itself the same way an explicit remove does.

The fix introduces a dedicated `download/storage.rs` module that owns both the
existing pre-allocation step (`prepare_game_storage`, moved out of
`planning.rs` because pure file I/O has no business sitting next to chunk
planning) and a new `discard_cancelled_download` sweep. The orchestrator
calls the sweep at every cancellation exit point, immediately after
`rollback_version_ini_transaction` so the version sentinel transients are
gone before the bulk deletion runs.

The sweep deliberately preserves a known set of names so a cancelled update
of an installed game does not destroy user-extracted files:

  - `local/`                  committed install directory
  - `.local.installing/`,
    `.local.backup/`          in-flight install transaction state, needed by
                              `install::recover_game_root` on next startup
  - `.lanspread.json`         per-game install intent log
  - `.softlan_game_installed` external softlan installer marker
  - `.sync/`                  external sync tooling

Everything else under the game root (the `.eti` archives, any nested payload
directories, partial chunk files) is removed, and the game root itself is
removed if it ends up empty. The set matches `should_ignore_game_child` in
`services/local_monitor.rs` minus the version.ini transients (which the
rollback step removes itself just before the discard runs).

Tradeoff worth knowing: this does NOT restore the pre-update `version.ini`
sentinel. `begin_version_ini_transaction` parks the existing sentinel as
`.version.ini.discarded`, and `rollback_version_ini_transaction` deletes
that file rather than renaming it back. The user-visible consequence is
that cancelling a mid-flight update of an installed game leaves the local
install playable but no longer flagged as "downloaded" — the documented
"settles as local-only" behaviour now recorded in
`crates/lanspread-peer/ARCHITECTURE.md` and `README.md`. Restoring the
sentinel on cancel was considered, but it would mean a cancelled update
keeps advertising the OLD version as Ready, which is worse than the
current outcome.

Two unrelated correctness issues that surfaced while threading cancellation
through the orchestrator are bundled in here because they belong to the
same user-visible "Cancel button works" story:

1. `download_from_peer` now races `connect_to_peer` against
   `cancel_token.cancelled()` (`download/transport.rs:314-322`). Previously
   a cancel arriving while QUIC was still in its connect handshake had to
   wait for the connect timeout to elapse before the cleanup could run.

2. The download task in `handlers.rs` now calls
   `refresh_local_game_for_ending_operation` on every terminal branch —
   success-without-install, install-handoff-failure, and the `Err(e)` /
   cancel branch — before `end_download_operation` clears
   `active_downloads`. Without this, the UI's settled snapshot on the
   cancel path could lag behind the actual file system state because the
   active-operation snapshot was cleared while the discard was still
   running, leaving a brief window where the card showed the pre-cancel
   state.

What this does NOT fix: a crash (process kill, power loss) during a
download still leaves orphan `.eti` files because `recover_download_transients`
in `install/transaction.rs` only sweeps the version.ini transients. Closing
that gap would mean calling the same discard from startup recovery for any
game root whose install intent is None and whose `version.ini` is absent.
Tracked in `FINDINGS.md` as a follow-up.

Test Plan:
- `just clippy && just test` — 102 unit tests pass, no new warnings.
- Two new storage tests:
  - `discard_cancelled_download_removes_peer_owned_payload` exercises the
    fresh-download cancel (no `local/`, root sweeps clean).
  - `discard_cancelled_download_preserves_local_install_state` exercises
    the update cancel (`local/`, `.lanspread.json`, `.local.backup/`
    survive; `version.ini` and `.eti` go away).
- Manual GUI smoke (operator): start a fresh download of a multi-archive
  game from a peer, click Cancel from the detail modal while the progress
  bar is between 5% and 95%. Expect the game root to be empty (or absent)
  afterwards and no orphan `.eti` files. Repeat against an installed game
  by clicking Update, then Cancel mid-download; expect `local/` contents
  intact and the card to drop back to Play (or Update if the newer-version
  peer is still around).
- `lanspread-peer-cli` has no `cancel` command yet, so the headless
  `PEER_CLI_SCENARIOS.md` matrix does not cover this end-to-end. Adding a
  CLI cancel command + scenario is the natural follow-up.

Refs: 62ceb06 (feat(peer): remove downloaded game files safely)
Refs: b7df2de (fix(download): emit failure events on early-returns and update UI transition)

crates/lanspread-peer/README.md

@@ -77,15 +77,17 @@ When the UI asks to download a game:
    different regions of the same file.
 5. `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 or
-   cancelled downloads sweep `.version.ini.tmp` and `.version.ini.discarded`
-   without restoring the previous sentinel.
+   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`
    is emitted and the peer auto-runs the install transaction.
 
 `PeerCommand::CancelDownload` cancels the tracked download token for an active
 transfer. The transfer task remains responsible for clearing `active_operations`,
-so the UI continues to treat active-operation snapshots as the single source of
+discarding partial payload files, and refreshing the settled local snapshot, so
+the UI continues to treat active-operation snapshots as the single source of
 truth for whether a download is still running.
 
 ### Install Transactions