apps/ui: Add dynamic post collection widgets to the desk UI

[youknowriad]

Summary

• Introduce a widget-domain resolver architecture so collection widgets can derive dynamic child widgets without exposing editor internals to widget definitions
• Add the post collection widget with loading, resolution, invalidation, and transparent rendering behavior that matches the prototype more closely
• Reuse a shared LoadingPlaceholder across post, page, and site preview widgets, and keep collection-derived widgets non-deletable in the toolbar
• Refine page loading so the placeholder sits inside the inset border and inherits the widget tone

Testing

• npm run typecheck
• npm test -- apps/ui/src/ui-desks
• Lint/format pass on the modified stylesheet files

[wpmobilebot]

📊 Performance Test Results

Comparing 30a73db vs trunk

app-size

Metric	trunk	30a73db	Diff	Change
App Size (Mac)	1408.82 MB	1408.82 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	30a73db	Diff	Change
load	1493 ms	1497 ms	+4 ms	⚪ 0.0%

site-startup

Metric	trunk	30a73db	Diff	Change
siteCreation	8092 ms	8096 ms	+4 ms	⚪ 0.0%
siteStartup	4922 ms	4928 ms	+6 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

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