Fix stale cache launches, FNF runtime deps, and uninstall focus death (v0.1.20)

- Inject the user-level shared-libs dir into LD_LIBRARY_PATH for installed
  games so FNF finds libvlc.so.5 (idempotent, avoids duplicate entries)
- Reconcile cached installs against butlerd's live caves: normalize
  int/float ids, wait for butlerd when the cache claims installs, keep the
  cache when Fetch.Caves fails, and re-save the reconciled cache
- get_caves() returns null on RPC error (vs [] for genuinely nothing
  installed) so callers can tell a transient failure apart
- Patch freshly installed/updated items in place so the Play button launches
  the new binary immediately
- Drop and re-add items through the LibraryManager so the library menu
  re-renders after install/uninstall (core handler is a no-op upstream)
- Restore launch-page focus after uninstall by searching get_tree().root:
  get_tree().current_scene is null in OGPU, so the old fix silently no-oped
  and left the D-pad dead

core/itch_client.gd

@@ -463,23 +463,26 @@ func _fetch_collection_games(profile_id: int, collections: Array) -> Array:
 
 
 ## Returns every installed game (a "cave" in butlerd terminology).
-func get_caves() -> Array:
+## Returns `null` when butlerd is unreachable or the call errored, so callers
+## can tell a genuine "no games installed" (`[]`) apart from a transient
+## failure.
+func get_caves() -> Variant:
 	return await thread_group.exec(_get_caves)
 
 
-func _get_caves() -> Array:
+func _get_caves() -> Variant:
 	var params := {}
 	var res := await _rpc_call("Fetch.Caves", params)
 	if "error" in res:
 		logger.warn("Fetch.Caves failed: " + str(res["error"]))
-		return []
+		return null
 	if res.get("stale", false):
 		logger.info("Caves are stale. Refreshing from itch.io.")
 		params["fresh"] = true
 		res = await _rpc_call("Fetch.Caves", params)
 		if "error" in res:
 			logger.warn("Fetch.Caves (fresh) failed: " + str(res["error"]))
-			return []
+			return null
 	return res.get("items", [])