Add webcam/camera support and self image capture plugin for Synth

[XargonWan]

Summary

Add a new non-LLM plugin to let Synthetic Heart interact with the scene camera, manage webcam state, and capture images from the virtual scene. This includes support for selfies, screenshots, photo sending, and a selfie-oriented animation pose state.

Motivation

Synth already has a digital body and animation system. Enabling real webcam/camera interactions closes the gap between virtual persona and scene-aware media actions, allowing:

• turning the webcam on/off
• capturing photos or screenshots
• sending image attachments
• triggering selfie poses before capture

Proposed Changes

1. New webcam/camera plugin

   • Create plugins/webcam_plugin.py (or similar)
   • Implement get_supported_actions() for actions like:
     • enable_webcam
     • disable_webcam
     • take_screenshot
     • take_selfie
     • send_photo
     • set_camera_pose

2. Session metadata persistence

   • Reuse session_meta.py for camera state storage
   • Persist webcam on/off state, current camera rect, and scene focus per interface_path
   • Ensure archive endpoints include camera metadata on save

3. Animation support

   • Extend animation_handler.py to support selfie/camera pose states
   • Register a new logical animation state or alias for selfie pose
   • Keep state-driven animation selection compatible with the existing handler

4. Action validation

   • Ensure the new actions are discovered by the auto-registration process in component_auto_registration.py
   • Add required field definitions for validation

5. WebUI / interface integration

   • Wire new camera metadata into WebUI session save/load flows
   • Optionally expose camera commands in the UI once backend support exists

Acceptance Criteria

• New webcam plugin exposes camera-related actions via get_supported_actions()
• Core validation accepts the new action types
• Camera/webcam state is stored and merged correctly through session_meta.py
• Archiving preserves camera metadata with session archives
• Selfie pose can be triggered through the animation state system
• Existing plugin registration flow automatically discovers the new plugin
• Add unit tests for action validation and session metadata flow

Notes

• This PR should remain focused on backend/plugin integration and metadata plumbing.
• Full WebUI controls for webcam toggling and photo capture can be scoped to follow-up work.
• Selfie pose support should use logical animation states, not hardcoded file paths.

Relevant files

• session_meta.py
• webui.py
• animation_handler.py
• component_auto_registration.py
• plugin_base.py
• plugins (new module)