feat(client): save/load client UI — F5/F6 quicksave/quickload (#554)

Wire SaveGame/LoadGame player actions through the full client stack:
protocol v15 decode, InputMapper F5/F6 bindings, SimBridge wire mapping
with one-shot carry-forward, GameState save_result field, and HUD
notification via monologue display. Quit-to-menu triggers quicksave
before scene change.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/data/ui-strings.yaml

@@ -104,6 +104,9 @@ notifications:
 
   # System
   save_complete: "Progress saved."
+  load_complete: "Session restored."
+  save_failed: "Save failed."
+  load_failed: "Load failed."
   connection_lost: "Signal interrupted."
   connection_restored: "Signal restored."