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-peer/src/lib.rs

@@ -83,7 +83,12 @@ pub use crate::{
     launch_settings::{LaunchSettingsOutcome, apply_launch_settings_once},
     startup::PeerRuntimeHandle,
     state_paths::{launch_settings_applied_path, setup_done_path},
-    stream_install::{NoopStreamInstallProvider, StreamInstallFuture, StreamInstallProvider},
+    stream_install::{
+        ExternalUnrarStreamProvider,
+        NoopStreamInstallProvider,
+        StreamInstallFuture,
+        StreamInstallProvider,
+    },
 };
 
 // =============================================================================