Add component contribution registry for composer and workspace controls

[IgorWarzocha]

Goal

Add a component/action contribution registry for composer and workspace controls so future features can add UI affordances without threading more bespoke props through large components.

Scope

• Identify the first stable contribution points for composer controls, footer controls, workspace toolbar actions, and settings sections.
• Register existing controls through typed descriptors where doing so reduces prop drilling.
• Keep rendering local React components; this is not remote plugin loading.
• Avoid changing extension APIs until the internal registry shape has settled.
• Keep the registry compatible with future server/capability-driven UI, including features that may depend on a Pi server being present.

Acceptance criteria

• At least one composer or workspace control group is driven by typed registry descriptors.
• Existing keyboard, dismissal, and focus behavior is preserved.
• The registry reduces direct coupling or prop drilling in the touched surface.
• Registry entries can declare required capabilities without coupling directly to Electron or Pi runtime internals.
• bun run ai:check passes.

[IgorWarzocha]

Part of #226.