feat(tauri): add low-disk streamed install action

NEXT_STEPS item 1 called out that streamed install was still CLI-only
because the Tauri app started the peer with no stream provider. Users can now
choose an explicit "Low disk install" action from the game detail modal for
remote-only games instead of taking the default archive-preserving download
path.

The GUI command queues a normal peer detail fetch first so the peer database
has the file metadata needed for source validation. A small pending handoff in
Tauri routes the resulting GotGameFiles event into StreamInstallGame instead
of DownloadGameFiles, and clears that pending state on no-peer or download
failure events. This keeps the existing download continuation untouched for
the default action.

The external unrar stream provider moved from the CLI harness into
lanspread-peer so CLI and Tauri use the same implementation. Tauri resolves
the bundled unrar sidecar path and injects that provider at peer startup;
falling back to the noop provider keeps peer startup alive if the sidecar
cannot be resolved, while the streamed install operation still fails safely.

Test Plan:
- just fmt
- just test
- just frontend-test
- just clippy
- just build
- git diff --check

Refs: NEXT_STEPS.md item 1

crates/lanspread-tauri-deno-ts/src/lib/gameState.ts

@@ -114,6 +114,12 @@ export const needsUpdate = (game: Game): boolean => {
     return (compareVersionStamps(game.eti_game_version, game.local_version) ?? 0) > 0;
 };
 
+export const canStreamInstall = (game: Game): boolean =>
+    !game.downloaded
+    && !game.installed
+    && game.peer_count > 0
+    && !isInProgress(game.install_status);
+
 /** What pressing the card's main action button should do, given the state. */
 export type PrimaryAction = 'play' | 'install' | 'update' | 'download' | 'busy' | 'disabled';