Feat/models and storage

[danieljtrujillo]

settings: add local-first model onboarding slice
settings: finish the noob-friendly model onboarding plan

Completes the remaining phases on top of the local-first slice:

• /api/storage/locations now carries a per-directory model inventory
  (HF cache repos, every .safetensors in the local model folders with
  the recommended pick starred ARC-over-RF and medium-over-small, torch
  hub checkpoints); the Locations size readout shows it on hover.
• /api/storage/checkpoints/inspect reports exactly what a path holds
  (checkpoints, configs, the precise pairing problem, recognized
  variant) and the Add flow runs it on every failure.
• /api/storage/checkpoints/generate-config copies the OFFICIAL config
  JSON next to a recognized built-in checkpoint from the local/cached
  copy; nothing is guessed and nothing downloads. The Add flow offers
  the button only when that is possible.
• CREATE and LOAD now refuse cleanly when no usable model exists or
  local-only blocks the selection: plain-language log line, run
  stopped, Settings opened with the Models section pulsed via the
  open-settings {section:models} event.
• TRAIN dataset path uses the shared PathInput with the native folder
  picker, gaining its missing label along the way.
• USER_GUIDE 21.1 + README rewritten for the final shape: local-only
  default ON, readiness cards, Browse pickers, config guidance, the
  no-model routing, and the full endpoint table.