UI: show player tank condition by remaining lives #6

Goal

The player tank should visually show a different condition at different remaining lives.

Scope

Prefer src/game.js only unless a small CSS class is clearly needed. Preserve existing DOM ids.

Requirements

At 3 lives, tank looks healthy/normal.
At 2 lives, tank shows damaged/intermediate condition.
At 1 life, tank shows critical condition.
Condition should update immediately after the player is hit and after restart.
Do not change core gameplay balance.

Files allowed

src/game.js
src/styles.css only if visual styling cannot be kept in canvas drawing

Files forbidden

index.html unless necessary for accessibility text
Git/config files

Acceptance checks

Player tank appearance differs for 3, 2, and 1 lives.
Restart returns tank to healthy state.
Existing HUD lives count remains correct.

Handoff notes

Agent: qoder
Branch: agent/qoder/player-condition-issue-N
Integration owner: codex

Codex orchestrator: assigning to QoderCLI in branch agent/qoder/player-condition-issue-6.

Codex orchestrator: integrated branch agent/qoder/player-condition-issue-6 into main via local verified merge commit a258454, then pushed main. Validation: node --check, git diff --check, browser smoke test with no page errors.