Add screenshot export for full world view

[AbdullahSAhmad]

Summary

• New canvas-export.ts module that captures all 9 zones as a PNG using Pixi v8's renderer.extract.canvas() API
• Screenshot button with camera icon added to top bar .tb-actions area (before notifications button)
• Day/night overlay temporarily hidden during capture for clear screenshots
• Camera transform reset during capture to ensure full world is captured regardless of current zoom/pan
• Double-click guard prevents multiple simultaneous captures
• Filename format: agent-move-screenshot-YYYY-MM-DDTHH-MM-SS.png

Test plan

• Click screenshot button — verify PNG downloads with correct filename
• Capture with day/night overlay active — verify overlay is excluded from screenshot
• Capture while zoomed/panned — verify full world is captured (not just viewport)
• Double-click rapidly — verify only one download occurs
• Capture with no agents — verify empty world captures correctly
• Capture with multiple agents in different zones — verify all visible

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