Implement combined FOV and skychart mode with horizon toggle

[giancarloerra]

Summary

This PR implements a combined mode for field of view (FOV) and skychart, including a toggle for horizon functionality and various UI updates.

Motivation

This change enhances user experience by integrating FOV and skychart features into a single mode, improving accessibility and usability.

Changes

Removes the Framing/Sky Chart mode toggle and merges both into a single unified view.

• All sidebar panels (Camera, Favorites, Time, Horizon) are now always visible
• Replaced mode toggle buttons in quick bar with a single Horizon toggle (globe icon)
• Horizon & Grid panel has a master "Enable Horizon & Grid" checkbox, synced with quick bar
• When horizon is off: no alt-az grid, horizon line, cardinal labels (framing-focused)
• When horizon is on: all horizon features per individual checkboxes (observation-focused)
• Time controls now work in both contexts (plan a shot at a future time)
• Backward compatible: old saved mode: 'skychart' auto-converts to horizonEnabled: true

Testing

• npm run lint passes
• npm run typecheck passes
• npm test passes
• Verified manually in dev / built production mode

Breaking changes

• No breaking changes

Screenshots (if applicable)

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By submitting this pull request I confirm that my contribution is made under the terms of the GNU Affero General Public License v3.0 or later.