feat: polish schema selector UI

[vitorhugo-dotnet]

Implements the approved schema selector UI polish spec and implementation plan.

Scope:

• ArrowUp/ArrowDown schema navigation with clamped active option
• Enter confirmation and Escape close behavior
• ARIA active-descendant plus scroll-into-view for keyboard navigation
• vertical-only dark styled schema scrolling and long-name truncation
• compact GitHub footer link through Wails BrowserOpenURL
• Windows ICO regenerated with transparent corners in 16/32/48/64/128/256 px frames

Verification:

• TDD RED runs confirmed the new keyboard, scrolling, and footer tests failed before implementation
• GitHub Actions run 31432218654: Test job passed (Go tests, frontend tests, TypeScript)
• GitHub Actions run 31432218654: Windows Wails build passed, executable verification passed, artifact upload passed
• built go-script-sql-runner.exe was downloaded and inspected; all six PNG icon frames from the transparent ICO are embedded byte-for-byte in the executable and each has corner alpha 0

No schema persistence/execution semantics were changed.