feat: Add toggle mechanism to enable/disable models from loading on demand (#9304)

* feat: add toggle mechanism to enable/disable models from loading on demand

Implements #9303 - Adds ability to disable models from being auto-loaded
while keeping them in the collection.

Backend changes:
- Add Disabled field to ModelConfig struct with IsDisabled() getter
- New ToggleModelEndpoint handler (PUT /models/toggle/:name/:action)
- Request middleware returns 403 when disabled model is requested
- Capabilities endpoint exposes disabled status

Frontend changes:
- Toggle switch in System > Models table Actions column
- Visual indicators: dimmed row, red Disabled badge, muted icons
- Tooltip describes toggle function on hover
- Loading state while API call is in progress

* fix: remove extra closing brace causing syntax error in request middleware

* refactor: reorder Actions column - Stop button before toggle switch

* refactor: migrate from toggle to toggle-state per PR review feedback