[Fix] Chat controls fade into light IDE themes

[zoomote]

​Created by Roomote. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

• Replaced translucent-white selector borders and hover fills with shared VS Code dropdown, toolbar-hover, and focus tokens across mode, API configuration, auto-approval, and generic dropdown triggers.
• Replaced the fixed white todo-delete overlay with the existing accessible, theme-aware alert dialog.
• Added cancel/delete interaction coverage, enabled/disabled selector branch coverage, and light/dark computed-style checks for hover and keyboard focus states.

Why this change was made

These controls were designed around dark backgrounds, causing borders and interaction feedback to fade into light IDE themes. This follow-up addresses the confirmed token bypasses identified during the Mermaid light-theme audit.

Impact

The affected selectors and todo-delete confirmation remain visible and consistent in light and dark themes without changing their behavior. Real extension screenshots remain unavailable because this environment exposes the docs site but not the VS Code extension UI.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!