apps/ui: Disable the default tldraw context menu on the desk canvas

[youknowriad]

Summary

• Disable tldraw’s built-in context menu in the desk canvas
• Prevent the browser’s native right-click menu on the canvas wrapper
• Keep the change scoped to the desk experience only

Testing

• Lint and formatting passed for the updated canvas file
• Typecheck passed
• Relevant tldraw adapter tests passed locally

[wpmobilebot]

📊 Performance Test Results

Comparing 35b9e9f vs trunk

app-size

Metric	trunk	35b9e9f	Diff	Change
App Size (Mac)	1408.80 MB	1408.80 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	35b9e9f	Diff	Change
load	1465 ms	1490 ms	+25 ms	⚪ 0.0%

site-startup

Metric	trunk	35b9e9f	Diff	Change
siteCreation	8083 ms	8087 ms	+4 ms	⚪ 0.0%
siteStartup	4930 ms	4942 ms	+12 ms	⚪ 0.0%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)