feat(client): add save/load game flow and move debug_overlay (#257, #561)

#257: Add Load Game screen to main menu with sorted save list, loading
overlay during quickload round-trip, and pending_load_path cross-scene
flow. F5/F6 quicksave/quickload were already wired.

#561: Move debug_overlay.gd from scripts/ui/ to ui/ for consistency
with all other UI components. Update scene and test references.

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

client/data/ui-strings.yaml

@@ -109,6 +109,7 @@ notifications:
   load_failed: "Load failed."
   connection_lost: "Signal interrupted."
   connection_restored: "Signal restored."
+  loading: "Resuming..."
 
 # ============================================================
 # KNOWLEDGE PANEL LABELS
@@ -183,6 +184,9 @@ menu:
   confirm_quit: "Unsaved progress will be lost."
   confirm_yes: "Yes"
   confirm_no: "No"
+  load_game_browse: "LOAD GAME"
+  load_game_back: "BACK"
+  load_game_empty: "No saves found."
 
 settings:
   audio_volume: "Volume"