Add in-game chat, bot fill for casual, and borderless fullscreen

- In-game chat: T=all, Y=team (race doubles as team), server-relayed and
  team-filtered via new ChatManager autoload; last 10 messages shown
  bottom-left (chat/chat_box.gd).
- Bot fill for casual (bots/): keeps each of the match's 2 offered races
  at a minimum of 7 total humans+bots, spawning/despawning reactively as
  players join/leave. Bots always fly their race's fighter and use
  negative peer_ids so they ride the existing networked-ship stack
  (spawning, loadout sync, health/position sync, bullet attribution) with
  no special-casing. Casual matchmaking now forms with just 1 real player
  queued instead of waiting for a second (matchmaking-api/).
- Borderless fullscreen with stretch scaling disabled instead of scaling
  the Steam-Deck-matched 1280x800 canvas up to fill PC monitors (which
  read as zoomed in) — PC monitors now reveal more world/HUD at native
  size instead. Reworked main_menu/team_select/chat_box to position via
  anchors relative to the real window instead of hardcoded coordinates.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

matchmaking-api/.env.example

@@ -1,6 +1,9 @@
 DATABASE_URL=postgresql+asyncpg://matchmaking:matchmaking@postgres:5432/matchmaking
 
 # Real-value targets per overview/bots.md (casual min 7v7) and overview/racesclasses.md (ranked 5v5).
+# CASUAL_TEAM_SIZE no longer gates when a casual match forms (bot fill means
+# 1 real player is enough — see queue_manager.py); it only caps how many real
+# players group into one casual match.
 CASUAL_TEAM_SIZE=7
 RANKED_TEAM_SIZE=5