Mixer: enlarge the inline insert buttons

[emaspa]

Summary

The inline N, B and cog controls were using 10px text with no minimum size or vertical padding. Give all nine controls across the two microphone strips and mix cards shared styles with 12px text, vertical padding and minimum 24 by 24 logical-pixel targets.

Add descriptive accessibility names. Labels, tooltips, colours, bindings and click behaviour stay unchanged. No version bump or packaging changes.

Fixes #74.

Verification

• Locked restore and Release build with warnings treated as errors and the native plugin host enabled: 0 warnings, 0 errors.
• .NET test suite on the branch based on main: 333 passed, 0 failed, 0 skipped.
• Desktop acceptance of the same UI change before isolating it from the Flatpak development branch: buttons measured 42 physical pixels tall at 1.75 display scale, equivalent to 24 logical pixels. Five mix cards fit in a roughly 1300px-wide window without clipping or horizontal overflow.
• Extreme layouts with more mix cards were not tested. Existing ellipsis handling remains in place for plugin names.

No manual change is needed because the documented controls and their behaviour are unchanged.