fix(ui): smooth transitions and align settings layout

[bobleer]

Summary

• Add a shared Web UI motion layer for hover, press, scene, navigation-panel, and modal transitions.
• Preload lazy scene and settings content, keep outgoing content mounted during exit, and avoid blank-frame flashes during page switches.
• Standardize settings headers, content gutters, scrollbar reservation, loading frames, and responsive alignment.
• Add focused coverage for scene state, settings transitions, modal exit presence, and loading/layout contracts.

Type and Areas

Type: UI/UX, regression fix

Areas: Web UI, desktop settings

Motivation / Impact

Previously, interaction feedback and page transitions were inconsistent. Some scene and settings switches appeared to flash or hard-jump because lazy content replaced the current UI before the incoming subtree was ready. Settings pages also shifted horizontally depending on scrollbar presence and contained page-specific spacing overrides.

This change establishes shared motion and layout contracts so current and future surfaces inherit consistent behavior by default. Reduced-motion preferences remain respected.

Verification

• pnpm run type-check:web — passed
• pnpm --dir src/web-ui run test:run src/app/components/NavPanel/NavPanelLayout.test.ts src/app/stores/sceneStore.test.ts src/app/scenes/settings/SettingsScene.test.tsx src/component-library/components/Modal/Modal.test.tsx src/infrastructure/config/components/common/ConfigPageLayout.test.tsx src/infrastructure/config/components/ReviewConfig.test.tsx src/infrastructure/config/components/ExternalSourcesConfig.test.tsx — 7 files, 65 tests passed
• pnpm run build:web — passed, including Monaco asset verification
• git diff --check — passed
• Manual browser verification: scene switching, settings-page switching, modal transitions, long/short settings-page alignment, and loading frames

Reviewer Notes

• Shared motion defaults live in src/web-ui/src/app/styles/motion.scss.
• Settings geometry is enforced by the common ConfigPageLayout and ConfigPageHeader frames.
• AI-assisted contribution; fully tested at the Web UI level.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable. No new user-facing strings were added.