upstreamable: frontend: Fix mobile navbar control colors

[skoeva]

These changes make mobile navbar controls inherit the app bar foreground color so they remain readable with custom themes.

Fixes: #638, #659

Summary

• Updates the mobile logo/sidebar button to use color="inherit"
• Updates the compact global search button to use color="inherit"
• Lets mobile app bar controls inherit the color already computed by the parent app bar from theme.palette.navbar
• Improves custom theme support for mobile app bar controls, including custom SVG logos that use currentColor

Testing

• Manually verified the mobile logo text inherits the navbar foreground color
• Manually verified the mobile search icon inherits the navbar foreground color

[Copilot]

Pull request overview

This PR adjusts mobile top bar controls to inherit the AppBar’s foreground color, ensuring the sidebar toggle/logo and global search icon remain readable under custom navbar themes (addresses #638 and #659).

Changes:

• Updated the mobile sidebar toggle/logo button to use color="inherit" instead of forcing theme.palette.text.primary.
• Updated the compact/mobile global search IconButton to use color="inherit" so the icon/text follows the AppBar foreground color.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
frontend/src/components/Sidebar/HeadlampButton.tsx	Makes the mobile sidebar toggle/logo button inherit the AppBar text color for better theme compatibility.
frontend/src/components/globalSearch/GlobalSearch.tsx	Makes the mobile/compact global search button inherit the AppBar text color for better theme compatibility.

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