[FEATURE] Relocate Layout Lock Button so it's not consuming vertical screen space

[denete]

What does this PR do?

Moved lock button to render in a new left border tabset using FlexLayout. This panel can collapse. The lock layout control can not be deleted from the border space as we don't want the user to get "stuck" without being able to unlock it.

This addresses #662.

Type of change

• Bug fix
• New feature
• Performance improvement
• Refactor / code cleanup
• Documentation
• Translation
• Map layer plugin

How to test

1. Select the dockable layout
2. Click the "Lock Layout" in the left border and then the "Lock Layout" button
3. Confirm that the button can lock and unlock the dockable layout

Checklist

• [✔] App loads without console errors
• [✔] Tested in Dark, Light, and Retro themes
• [✔] Responsive at different screen sizes (desktop + mobile)
• If touching server.js: caches have TTLs and size caps (we serve 2,000+ concurrent users)
• If adding an API route: includes caching and error handling
• [~] If adding a panel: wired into Modern, Classic, and Dockable layouts
• [✔] No hardcoded colors — uses CSS variables (var(--accent-cyan), etc.)
• [✔] No .bak, .old, console.log debug lines, or test scripts included

Screenshots (if visual change)