Add screenshot export feature

[AbdullahSAhmad]

Summary

• Adds a new Screenshot Export feature that captures the Pixi.js canvas as a PNG image
• Supports viewport (current view) and full-world (all 9 zones) capture modes via a toggle in the preview modal
• Preview modal shows the captured image with Download PNG and Copy to Clipboard actions
• Accessible via Shift+E keyboard shortcut, command palette entry, and camera button in zoom controls

Files Changed

• NEW packages/client/src/ui/screenshot-export.ts — Core capture logic + preview modal (210 lines)
• packages/client/src/main.ts — Wire up module, action handler, keyboard shortcut, cleanup
• packages/client/src/ui/command-palette.ts — Add screenshot command entry
• packages/client/src/ui/shortcuts-help.ts — Add Shift+E shortcut documentation
• packages/client/index.html — Add camera button to zoom controls
• packages/client/styles/modals.css — Add screenshot export modal styles

Technical Details

• Viewport capture: Uses canvas.toBlob() for direct canvas-to-PNG conversion
• Full-world capture: Temporarily resets camera to fit-to-view, renders a frame, captures, then restores camera state
• Clipboard: Uses navigator.clipboard.write() with ClipboardItem for image copy; falls back to opening in new tab
• Follows existing patterns: modal structure matches SessionExport, CSS prefix sse-, consistent z-index

Test plan

• Press Shift+E to open screenshot modal — verify viewport preview appears
• Toggle to "Full World" — verify all 9 zones are visible in preview
• Toggle back to "Viewport" — verify it recaptures current view
• Click "Download PNG" — verify PNG file downloads with correct filename
• Click "Copy to Clipboard" — verify image can be pasted
• Click camera button in zoom controls — verify modal opens
• Open command palette (Ctrl+K), search "screenshot" — verify command appears and works
• Press ? — verify Shift+E shortcut is listed in help
• Close modal via backdrop click, X button, and Escape key

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