fix(#386): strengthen multiselect Apply states

[BorisTyshkevich]

What & why

Closes #386.

Restores the multi-select Apply button as a clear primary action by explicitly styling its hover, keyboard-focus, pressed, and disabled states. Adds a browser harness that exercises those states in both supported themes.

Checklist

• npm test passes (the per-file coverage gate is non-negotiable)
• Tests added/updated in the same change as the code
• npm run build succeeds (single-file dist/sql.html)
• Layers kept honest: pure logic in src/core/, network in src/net/ (injected fetch), DOM in src/ui/
• No new runtime dependency (or it's a deliberate, justified addition — see CONTRIBUTING)
• README / CHANGELOG.md ([Unreleased]) updated if behavior or the deployed surface changed
• Reconciled affected tracked work (roadmap Roadmap to 1.0.0 #68, the issue body, ADR/CHANGELOG) if this change reshaped it

Verification

• npm test — passed
• npm run build — passed
• npx playwright test tests/e2e/multi-select.spec.js — unable to launch browsers locally: all engines exit with SIGTRAP before test execution; the new test is included for CI.