feat: add context settings controls to WebUI

[Activer007]

Summary

• add WebUI controls for current_input_file inline threshold and filename policy
• add editable Context Engine mode/strategy controls with env override handling
• persist context_engine mode/strategy via PUT /admin/settings and cover it with admin settings tests
• document context_engine as writable in API docs

Verification

• ./scripts/lint.sh
• ./tests/scripts/check-refactor-line-gate.sh
• ./tests/scripts/run-unit-all.sh
• npm run build --prefix webui