Refactor web App.tsx into smaller modules

[codetea-ping999]

Goal

Reduce maintenance risk in the Studio web app by splitting apps/web/src/App.tsx into smaller modules with clear responsibilities.

Scope

• apps/web/src/App.tsx
• New files under apps/web/src/components/
• New files under apps/web/src/hooks/
• New files under apps/web/src/lib/

Background

App.tsx currently handles API calls, polling, payload building, gallery actions, asset detail handling, and rendering. The feature set is useful, but the file has become a high-change concentration point.

Required changes

• Move API request helpers into apps/web/src/lib/api.ts
• Move request/reuse payload builders into apps/web/src/lib/payloads.ts
• Move job polling logic into apps/web/src/hooks/useJobPolling.ts
• Extract presentational pieces into components such as:
  • StagePreview
  • gallery panel/list
  • asset detail panel
• Keep current UI behavior and endpoint usage intact
• Prefer low-risk refactoring over redesign

Constraints

• Do not change visible product behavior unless necessary for correctness
• Keep endpoint contracts unchanged
• Avoid introducing new dependencies unless clearly justified

Acceptance criteria

• npm run build passes in apps/web
• App.tsx becomes a composition/root container instead of a large mixed-responsibility file
• Existing generation, polling, gallery selection, reuse, export, and project binding flows still work

Validation

• Run cd apps/web && npm run build
• Manually smoke-check:
  • media type switching
  • queue generation
  • latest job polling
  • gallery item selection
  • asset reuse/export/project binding