feat: replace theme select with icon button group

[mondalaci]

Summary

• Replace the Settings theme ng-select with a Bootstrap button group using desktop / sun / moon icons
• Keep hover tooltips for the previous theme labels and dismiss them on click
• Remove unused AppThemeSelect / getSupportedThemes plumbing

Test plan

• Open Agent Settings and confirm the theme control is an icon button group
• Hover each button and verify the tooltip text (Follow OS / Light / Dark)
• Click each theme and confirm it applies and the tooltip does not stick
• On Linux, confirm the system-theme circle tooltip still appears with spacing after the buttons

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