Update default chat model from sonnet to a configurable default

[vybe]

Problem

The default chat model is hardcoded to sonnet (Sonnet 4.5) in docker/base-image/agent_server/services/claude_code.py:59:

def get_default_model(self) -> str:
    """Get default Claude model."""
    return "sonnet"  # Claude Sonnet 4.5

Meanwhile, the frontend ModelSelector.vue labels Opus 4.5 as "Default — most capable", creating a mismatch between what the UI suggests is the default and what actually runs.

Expected Behavior

• The backend default model should match what the UI advertises as default, or
• The default should be configurable at the platform level (not just per-agent)

Suggested Fix

1. Add a platform-level setting for the default chat model (e.g., in Settings or a config file)
2. Update get_default_model() to read from that setting, falling back to a sensible default
3. Align the frontend ModelSelector "Default" label with the actual backend default

Current Model Selection Priority

1. Per-request model field in chat payload
2. Session-level state (set via PUT /api/agents/{name}/model)
3. Environment variable (AGENT_RUNTIME_MODEL or CLAUDE_MODEL)
4. Hardcoded default (sonnet)

Files Involved

• docker/base-image/agent_server/services/claude_code.py — get_default_model()
• src/frontend/src/components/ModelSelector.vue — preset list and "Default" label
• docker/base-image/agent_server/state.py — current_model initialization