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

NEXT_STEPS.md

@@ -5,11 +5,13 @@ archive-derived install bytes into `local/` without making the receiver a
 source?” Yes. Next I’d harden the pieces that decide whether this is
 product-ready.
 
-1. **Move from CLI-only to real app integration**
+1. **Done — Move from CLI-only to real app integration**
 
-   Add a GUI command/control path for “stream install / low disk mode”,
-   probably behind an explicit option. The Tauri crate currently opts out with
-   `stream_install_provider: None`, so the GUI cannot use it yet.
+   The GUI now has an explicit “Low disk install” action in the game detail
+   modal for remote-only games. The Tauri backend queues that path through
+   `stream_install_game`, injects the shared external `unrar` stream provider,
+   and hands fetched file details to `StreamInstallGame` instead of the normal
+   download command.
 
 2. **Replace per-file `unrar p` with a final archive provider**