Add comprehensive bot publishing workflow to marketplace with GitHub auth
and Docker image building. Introduce avatar generation from style refs
using xAI Grok Imagine.

Key changes:
- `publish_marketplace` tool with build/submit_to_review modes
- `generate_avatar` tool with style bank seeding and idea-based generation
- Backend GraphQL mutations: `botwiz_marketplace_action`, avatar RPCs
- Frontend marketplace action menu and OAuth popup handling
- Improved expert reuse by fexp_id and provenance tracking
- Style bank manifest and default assets in flexus-client-kit

Introduce _FULLSIZE_TARGET (1024x1536) and resize logic that:
- Scales images proportionally to fit within target dimensions
- Centers the scaled image on a transparent canvas
- Applies to both WebP conversion and direct file saving

Ensures consistent output dimensions while preserving aspect ratio.

Introduce _encode_webp_within_limit() to progressively reduce quality
from 100 down to 40 until image fits within 250KB limit, preventing
upload failures due to oversized files.

Replace direct WebP saves with this function in fullsize and 256px
avatar generation.

Make xai_sdk a hard dependency by removing the try/except import and
related runtime check. Users must now install xai-sdk explicitly.

- Add continuous heartbeat loop (every 10s) in service_devenv.py
- Replace sys.pong with sys.heartbeat messages
- Update frontend WebSocket to use heartbeat timeout (30s) instead of ping/pong
- Remove ping/pong handling and auto-restart logic from botwiz.vue
- Add heartbeat task lifecycle management and proper cleanup
- Update TypeScript types for new heartbeat protocol