Feat(plugin): Add set camera focus as ui-command (v0.1.x)

[imdt-claudiop]

Adds setCameraFocus to the camera UI commands so plugins can focus or clear a camera by user ID or stream ID:

• src/ui-commands/camera/types.ts: webcamSelector union (ByUserId / ByStreamId), SetCameraFocusCommandArguments, and the setCameraFocus method on UiCommandsCameraObject
• src/ui-commands/camera/enums.ts: SET_CAMERA_FOCUS_COMMAND
• src/ui-commands/camera/commands.ts: typed dispatcher for the command
• tests/unit/ui-commands/camera/commands.test.ts: unit coverage for the userId selector, streamId selector, and focus: false payloads (asserting the enum and exact CustomEvent detail)
• README.md: documents the command and its typed usage

This is the same API proposed in #168, carried onto the current v0.1.x line (0.1.26) so 4.0 clients can consume it - #168's branch is 181 commits behind v0.0.x and does not build against the current plugin engine. The client-side listener ships in a sibling bigbluebutton PR pinned to this branch's commit.

Validation

• npm run test:unit (34 tests, including the new camera command suite)
• npm run lint, npm run build

Consumers on the 3.0 line: the equivalent v0.0.x carry is available as branch claudio/camera-focus-v0.0.x on the same fork.