Add comprehensive UI system with action bar, unit frame, and character sheet

- Created UI system in level/ui/ folder
  - Action bar with 12 ability slots (Attack, Block, Dash, Jump + 8 expansion slots)
  - Unit frame showing player portrait with health bar
  - Character sheet/spellbook (toggle with Tab) displaying stats, weapons, and abilities
  - Tab hint indicator showing how to open character sheet
  - Custom theme with golden borders and dark backgrounds

- UI Components:
  - HUD Manager autoload handles all UI initialization and player connections
  - Ability buttons with cooldown overlay and keybind display
  - Real-time health and cooldown tracking via signals
  - Scrollable character sheet with two-page layout

- Player Integration:
  - Added UI signals: dash_cooldown_updated, attack_cooldown_updated, weapon_equipped_changed
  - Mouse capture system (Escape to toggle, click to recapture)
  - Synced attack cooldown from weapons to player for UI tracking

- Updated level.gd to connect local player to HUD Manager
- Updated CLAUDE.md with git commit guidelines

level/scripts/base_weapon.gd

@@ -44,6 +44,10 @@ func perform_attack() -> bool:
 
 	_attack_timer = weapon_data.attack_cooldown
 
+	# Notify owner character of attack cooldown (for UI)
+	if owner_character and owner_character.is_multiplayer_authority():
+		owner_character._attack_timer = weapon_data.attack_cooldown
+
 	# Play attack animation on owner
 	if owner_character._body:
 		owner_character._body.play_attack()