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/README.md

@@ -33,10 +33,15 @@ rebuilding the image. Rebuild (`docker compose up -d --build`) only when
   returns the assigned server's `server_ip`/`server_port`/`team`.
 - `POST /matchmaking/queue/leave?ticket_id=...` — cancel a queued ticket.
 
-A background loop (`app/queue_manager.py`) runs every 2s, and once enough
-players are queued for a mode (`CASUAL_TEAM_SIZE`/`RANKED_TEAM_SIZE` × 2) it
-forms a match, splits them into two teams, and assigns the first available
-`GameServer` for that mode.
+A background loop (`app/queue_manager.py`) runs every 2s and forms a match
+once enough players are queued for a mode, then splits them into two teams
+and assigns the first available `GameServer` for that mode. Casual and
+ranked differ here: casual has bot fill (`spacewar/bots/bot_manager.gd`
+pads both teams up to `GameConfig.bot_min_team_size` in-game), so 1 queued
+player is enough to form a casual match — `CASUAL_TEAM_SIZE` just caps how
+many real players can group into one match, it no longer gates formation.
+Ranked has no bots, so it still waits for the full `RANKED_TEAM_SIZE × 2`
+real players before forming.
 
 There's no real game-server pool yet — one dev server is auto-seeded on API
 startup from `DEV_SEED_SERVER_IP`/`DEV_SEED_SERVER_PORT` (defaults to