Restore original core parity in Electron app

[w2968066]

Summary

• Continues with the Electron/FastAPI direction from the PR, but audits it against the original Tkinter core behavior.
• Fixes the body_dirs/body_dir mismatch that broke preflight, benchmark, and render dispatch.
• Adds a real preflight endpoint so preflight checks capacity without starting rendering.
• Makes output directory optional and derives a safe default output folder when blank, matching original behavior expectation.
• Persists renderer config in localStorage and backend media/history under AppData so reinstalling does not reset user state.
• Restores resolution quality tag display and constrains the Electron window/font scaling to avoid clipped controls.

Original parity audit

• Preserved: recursive media scan, matrix routing, hook/body/BGM slicing, usage history clear, NVENC fallback, CFR/audio alignment, static watermark loop handling.
• Restored/fixed: body pool routing contract, preflight semantics, benchmark config path, optional output folder, config persistence, resolution tag, fixed-size window behavior.

Verification

• py -m py_compile backend/app/core/video_matrix.py backend/app/services/task_service.py backend/app/api/routes.py backend/app/models/schemas.py
• npm ci
• npm run build