fix(dashboard): provide plugin context to widgets

[DavidBabinec]

Fixes #379

Summary

• Preserve plugin version and granted permissions when dashboard widgets register.
• Mount grid, Customize mode, and Block Library renderers through one shared plugin context provider.
• Reuse that provider for panels, app pages, and canvas overlays, and document the widget hook contract.

Impact

Plugin dashboard widgets can now use plugin identity, settings, scoped routes, commands, and permission-gated host hooks from every dashboard render location.

Verification

• Broken-state repro: widget context rendered as ||||.
• Browser retest: context worked in the library, Customize mode, normal view, and scoped route call.
• Source-removal control: regression test failed with CONTROL_EXIT=1.
• React Doctor: no issues; build and lint exited 0.
• bun test: 6,728 passed, 0 failed, exit 0.