fix(vscode): move mode selector next to model dropdown and update styling

[awhite0030]

Description

Brief description of what this PR does

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Changeset

• Added a changeset (pnpm changeset) describing this change for the changelog

Docs-only or internal chores need no changeset (or run pnpm changeset --empty to note that intentionally).

Testing

Automated Tests

• New features include passing tests in .spec.ts/tsx files
• All existing tests pass (pnpm test:all completes successfully)
• Tests cover both success and error scenarios

Manual Testing

• Tested with Ollama
• Tested with OpenRouter
• Tested with OpenAI-compatible API
• Tested MCP integration (if applicable)

Checklist

• If this was for an open issue, I was assigned to it
• Code follows project style guidelines
• Self-review completed
• Documentation updated (if needed)
• No breaking changes (or clearly documented)
• Appropriate logging added using structured logging (see CONTRIBUTING.md)

---

Root cause: The mode selector was positioned on the far right due to the flex-1 class on the model selector stretching it, and it had a truncated max-width and a gear icon which lacked clarity.
Fix: Removed flex-1 from the model-trigger button so the mode-trigger (composer-settings-trigger) can immediately follow it. Replaced the mode-trigger gear SVG with a dropdown chevron and removed the max-w-[40%] from its label for clearer grouping on the left side of the input.
Validation: Ran pnpm run build, pnpm test:format, pnpm test:lint, pnpm test:types, pnpm test:knip, and targeted pnpm test:ava "./plugins/vscode/src/**/*.spec.ts" successfully.

Fixes #1095

Fixes #1095