feat(peer): remove downloaded game files safely

Downloaded but uninstalled games can still occupy significant disk space. Add a
separate removal path for that state instead of overloading uninstall, which is
reserved for deleting only `local/` installs.

The peer runtime now exposes `RemoveDownloadedGame` with matching lifecycle
and active-operation events. The filesystem delete is intentionally strict: the
id must be a catalog game and a single path component, the target must be a
direct child of the configured game directory, the root must not be a symlink,
it must have a regular root-level `version.ini`, and it must not contain
`local/`, `.local.installing/`, or `.local.backup/`. Only then do we recursively
remove the game root.

The Tauri bridge exposes this as `remove_downloaded_game`, the frontend shows a
matching danger action only for downloaded-but-uninstalled games, and a
confirmation dialog warns that re-downloading can take a long time.

Test Plan:
- git diff --check
- just fmt
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= just test
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= just clippy
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= just build

Refs: user redesign nitpick about removing downloaded uninstalled games

crates/lanspread-tauri-deno-ts/src/styles/launcher.css

@@ -295,7 +295,7 @@
   padding: 4px;
   background: var(--bg-3);
   border: 1px solid var(--bd-2);
-  border-radius: 10px;
+  border-radius: 8px;
   box-shadow:
     0 16px 40px -8px rgba(0, 0, 0, 0.5),
     0 0 0 1px rgba(255, 255, 255, 0.04);
@@ -1060,6 +1060,42 @@
   flex: 1;
 }
 
+.modal.confirm-modal {
+  width: min(420px, 100%);
+  padding: 28px;
+  background: var(--bg-2);
+  border-radius: 8px;
+}
+.confirm-icon {
+  display: inline-grid;
+  place-items: center;
+  width: 42px;
+  height: 42px;
+  border-radius: 999px;
+  color: #f87171;
+  background: rgba(239, 68, 68, 0.1);
+  border: 1px solid rgba(239, 68, 68, 0.22);
+}
+.confirm-modal h2 {
+  margin: 18px 0 10px;
+  font-size: 19px;
+  line-height: 1.2;
+  color: var(--t-1);
+}
+.confirm-modal p {
+  margin: 0;
+  color: var(--t-2);
+  font-size: 14px;
+  line-height: 1.5;
+}
+.confirm-actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: 10px;
+  margin-top: 22px;
+  flex-wrap: wrap;
+}
+
 /* Settings dialog */
 .settings-modal {
   width: min(640px, 100%);