Fix light mode popover backgrounds

[Dimillian]

Motivation

• Popover and dropdown backgrounds used hard-coded dark rgba values which produced dark backgrounds with dark text in light mode, making menus unreadable.
• Switch these UI elements to use the theme surface variables so they respect both light and dark color schemes.

Description

• Replaced hard-coded dark backgrounds with theme variables in src/styles/main.css for the worktree popover, workspace branch dropdown, and branch pill hover states (--surface-command, --surface-card-strong, --surface-control-hover).
• Replaced the workspace add menu background in src/styles/sidebar.css to use --surface-command so add menu respects the light theme.
• Changes are limited to CSS only and update how popovers/menus derive their background color from the theme.

Testing

• Launched the dev server with npm run dev -- --host 0.0.0.0 --port 4173 and confirmed the app served successfully.
• Ran a headless Playwright script to open the app and capture a screenshot of the workspace add menu, which completed and produced an artifact (screenshot) successfully.
• No automated unit tests were run because this is a UI-only CSS change.

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Codex Task