Add Nuke Wars gamemode

- Add NukeWars gamemode with team-based nuclear warfare
- Implement 3-minute prep phase for SAM defenses
- Add unit restrictions (only nukes and SAM launchers)
- Enforce Baikal map requirement
- Add territory-based win condition (<5% = loss)
- Add client-side timer UI
- Add tests for restrictions and win conditions

docs/gamemodes/nuke-wars.md

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+# Nuke Wars Gamemode
+
+Nuke Wars is a team-based nuclear warfare gamemode where teams battle for territory control using only nuclear weapons. The mode emphasizes strategic placement of defensive structures during a preparation phase, followed by an all-out nuclear assault.
+
+## Game Rules
+
+### Map and Teams
+
+- Only available on the Baikal map
+- Teams are split 50/50 on the map (Team 1 on left, Team 2 on right)
+- Each team starts in their own territory and cannot build across the midpoint during prep phase
+
+### Preparation Phase
+
+- 3-minute preparation phase at start
+- Teams can only build in their territory during this phase
+- Focus on setting up SAM defenses and missile silos
+- Countdown timer shows remaining prep time (3:00 -> 0:00)
+
+### Unit Restrictions
+
+- **Allowed Units:**
+  - Missile Silos: For launching nuclear weapons
+  - SAM Launchers: For missile defense
+  - Atom Bombs: Basic nuclear weapon
+  - Hydrogen Bombs: Advanced nuclear weapon
+- **Blocked Units:**
+  - MIRV missiles
+  - All conventional military units
+  - Cities and infrastructure buildings
+  - Ships and transport units
+
+### Combat Phase
+
+- After prep phase ends, nuclear warfare begins
+- Only nuclear missiles can cross the midpoint
+- SAM defenses remain territory-bound
+- SAM effectiveness:
+  - 100% chance to intercept Atom Bombs
+  - 80% chance to intercept Hydrogen Bombs
+
+### Win Conditions
+
+1. **Territory Control**
+   - A team loses if their territory drops below 5% of total map area
+   - Territory percentage excludes irradiated (fallout) tiles
+2. **Time Limit**
+   - If time limit expires, team with most territory wins
+
+### Strategic Elements
+
+- Balance between offense (missiles) and defense (SAMs)
+- Careful placement of SAM coverage
+- Managing nuclear strikes to maintain territory control
+- Protecting key defensive infrastructure
+
+## Setup Requirements
+
+- Gamemode available in both public and private lobbies
+- Must select Baikal map
+- Teams must be balanced
+- Settings automatically enforced when Nuke Wars is selected