Sticky settings and about menu items

[mondalaci]

Summary

Closes #2307.

Replace the collapsible Agent section at the bottom of the side menu with a sticky footer bar containing quick-access buttons for agent pages. The main menu (device, keymaps, macros, etc.) scrolls independently while the footer stays pinned.

• Remove the expandable Agent menu and its toggler state
• Add a sticky footer with Settings, About, and separators between buttons
• Restructure the side menu layout so the top section scrolls and the footer remains fixed

Note on Help: The original issue mockup only shows Settings and About buttons. Help is intentionally included as a third sticky button to preserve access to the existing Help page, which would otherwise be removed along with the Agent section. This is a deliberate extension beyond the mockup.

Test plan

• Open the app and verify the side menu footer shows Settings, Help, and About with icons
• Scroll a long keymap/macro list and confirm the footer stays visible at the bottom
• Click each footer button and verify navigation to /settings, /help, and /about
• Confirm the active button is highlighted on each page
• Confirm footer buttons are disabled while firmware is updating
• Verify button labels and icons are fully visible (no clipping or ellipsis)

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