fix(desktop): isolate runtime plugin render hooks

[embwl0x]

Summary

• mount runtime contribution render callbacks as real React child components
• apply the boundary consistently to slots, chat routes, route tiles, panes, and statusbar contributions
• add hot-registration and render-failure regressions

Root cause

Desktop hosts called plugin render() callbacks while rendering the host component. A hook used by a plugin therefore became a hook of the host. Loading or replacing a plugin changed the host's hook count and could trigger React error #310. It also meant a render-time plugin exception occurred before ContribBoundary received its child, so the boundary could not contain the failure.

ContribRender passes the callback to createElement, giving each contribution its own component boundary. Replacing a callback now remounts that child instead of changing the host's hook order, and plugin exceptions occur below ContribBoundary.

I audited every production .render() contribution call under apps/desktop/src; no direct callback invocation remains.

Fixes #80560

Verification

• npx vitest run --project ui src/contrib/react/slot.test.tsx - 2 passed
• npm run typecheck - passed
• npm run lint - 0 errors (89 pre-existing warnings)
• npm run build - passed, including assert-dist-built
• git diff --check origin/main...HEAD - passed
• contribution publish gate and gitleaks - passed

The full Desktop UI run completed with 3,624 passing tests and 8 aggregate timeout/order failures in four unrelated existing files. Each affected file passes in a fresh process (19/19 total); the intermittent messaging file also passes on untouched exact-base main (7/7).