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>

overview/bots.md

@@ -13,9 +13,26 @@
 - Avoids the ghost-town feeling of a 25v25 map with 3 players
 - Bots are not added beyond 7v7 so human players always dominate strategy
 
-## Implementation Notes (not yet built)
+## Implementation Notes
 
-- Bots use simplified AI: thrust toward nearest enemy, shoot when in range
-- Bot difficulty: intentionally easy (this is casual — bots are filler, not challenge)
-- Bot names: procedurally generated callsigns so they're not obviously bots in the HUD
-- Server tracks human vs bot count per team and manages swap-in on player join
+- **`bots/bot_manager.gd`** (server-authoritative autoload) owns bot lifecycle. All
+  spawn/despawn decisions funnel through one function, `_reconcile_race(race_id)`,
+  triggered by `PlayerRegistry`'s existing `loadout_updated`/`player_removed`
+  signals (join/leave) and `World.decide_offered_races()` (match start) — no
+  polling. This is the one place to change for future tweaks (fill curve, per-map
+  team size, etc.)
+- Bots get **negative peer_ids** (`-1, -2, ...`), assigned by `BotManager`. This
+  lets them reuse every existing networked-ship system for free — spawning
+  (`MultiplayerSpawner`), loadout replication/backfill (`PlayerRegistry`),
+  position/health/visibility sync, bullet damage attribution — none of which
+  cares whether a `peer_id` came from ENet or from `BotManager`
+- Bots always fly **the race's fighter/Interceptor** (`race.ships[0]` in
+  `team_select.gd`'s `RACES` data)
+- AI (`bots/bot_ai.gd`, `class_name BotAI`): thrust toward nearest living enemy,
+  turn to face it, shoot when roughly aimed and in range — simplified
+  seek-and-shoot, intentionally easy (casual bots are filler, not a challenge).
+  Fully isolated from `ship_movement.gd`'s networking code so future
+  difficulty/behavior tuning only ever touches this one file
+- Bot names: `bots/bot_names.gd`, a flat pool of procedural callsigns
+- Tunables live in `GameConfig` (`bot_min_team_size`, `bot_engage_range`,
+  `bot_stop_distance`, `bot_aim_tolerance_deg`)