fix(skill): correct apps.json documentation (#72)

## Summary
- apps.json is an **OBJECT**, not an array (was incorrectly documented)
- Groups must include `name` field with "Group N" pattern
- Groups use `type: "param"`, not `type: "endpointParam"`
- Added `/apps.json` endpoint to backend structure example
- Documented layout item `state` object for pre-configuring widgets
(chartView, chartModel, columnState)
- Documented single unnamed tab pattern (empty string id/name for no tab
bar)
- Added img, img_dark, img_light fields to apps.json structure

## Test plan
- [ ] Review the updated documentation in SKILL.md, OPENBB-APP.md, and
DASHBOARD-LAYOUT.md
- [ ] Verify the apps.json examples match the actual OpenBB Workspace
schema
- [ ] Test building an app using the updated skill documentation

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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>