feat: show avatar tiles for participants without a camera

[JoVictorNunes]

What does this PR do?

Adds avatar tiles to the Picture-in-Picture view for participants without an active camera, complementing the webcams already shown. This way every participant in the meeting stays visible in PiP, even those not sharing video.

Changes

• New USERS_SUBSCRIPTION (queries.ts): fetches users with isSharingCamera = false, ordered by name (limited to 10), including avatar, color, and talking state (voice.talking).
• New useUsers hook (hooks.ts): exposes the subscription via useCustomSubscription.
• New AvatarItem component (avatar-item.tsx): renders the user's avatar image or, when missing, a circle with the name initial and the user's color. Uses a ResizeObserver to hide the name when the tile gets narrow (< 140px).
• Grid integration (component.tsx): new AvatarMedia type in the GridMedia union; avatars are appended to the webcams (tiles = [...webcams, ...avatars]) across all scenarios (screenshare, slide, slide loading, and default).
• Styles (stylesheet.ts): .pip-avatar-item and .pip-avatar-circle classes, an avatar-pulse animation, and a blue border to indicate the talking user. Container background adjusted from #000 to #111.

How to test

1. Join a BigBlueButton session with the plugin enabled.
2. Have at least one participant without a camera turned on.
3. Open Picture-in-Picture and confirm the participant shows up as an avatar tile (image or initial with the user's color).
4. Verify the tile pulses / gains a blue border when the user is talking.
5. Shrink the tile width and confirm the name gets hidden.

Closes Issue(s)

Closes none

Motivation

• Users cannot know who is talking when the people talking are not sharing camera.