fix(app): avoid shortcut settings flash

[opencode-agent]

Summary

• use controlled tab state in both the legacy and V2 settings dialogs
• wrap every settings tab navigation in Solid's startTransition so the current tab remains painted while the next tab renders

Validation

• bun typecheck (packages/app)
• Prettier check for settings-keybinds.tsx
• Playwright at 1280×800 using the real app UI with mocked backend data
• Playwright navigated General, Shortcuts, Servers, Providers, and Models with 188 animation-frame samples and no blank frames or errors
• Synchronized before/after video

Comparison

• before: b44bc0ad0740afe80beae0a1981c2c4d96f67173
• after: 86e4358319e10097124ce704d4c70c409d23f167