Share the desk shell across user and site desks

[youknowriad]

Summary

• Introduced a shared Desk component that handles the full desk shell for both user and site desks.
• Kept the behavioral differences behind siteId?: string:
  • user desk persists to desks.user and keeps the welcome note default
  • site desks persist to desks.sites[siteId] and start empty
  • chats are scoped by siteId when present
• Moved the shared desk canvas, action context, and tldraw adapter under apps/ui/src/ui-desks/desk/.
• Added site desk IPC/query support plus tests for site desk persistence.
• Kept the header plus menu registry-driven so widgets can be added from the same menu on both desk variants.

Testing

• npx eslint --fix on the touched files passed.
• npm run typecheck passed.
• npm test -- apps/ui/src/ui-desks/widgets/create-widget.test.ts apps/ui/src/ui-desks/desk/tldraw-adapter.test.ts apps/studio/src/modules/desks/lib/ipc-handlers.test.ts passed.

[youknowriad]

I'm not sure we need this at the moment. Let's assume it's true for all widgets

[wpmobilebot]

📊 Performance Test Results

Comparing 8d84eb6 vs trunk

app-size

Metric	trunk	8d84eb6	Diff	Change
App Size (Mac)	1408.80 MB	1408.80 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	8d84eb6	Diff	Change
load	1501 ms	1494 ms	7 ms	⚪ 0.0%

site-startup

Metric	trunk	8d84eb6	Diff	Change
siteCreation	8092 ms	8096 ms	+4 ms	⚪ 0.0%
siteStartup	4934 ms	4930 ms	4 ms	⚪ 0.0%

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Results are median values from multiple test runs.

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