* Studio: live model-load progress + rate/ETA on download and load
Two UX fixes for the opaque multi-minute wait between clicking Load
and being able to chat, visible most clearly on large MoE GGUFs like
MiniMax-M2.7 (131 GB of weights on a 97 GB GPU):
1. **Model-load phase is now observable.** The existing chat flow
transitions the toast to "Starting model..." as soon as the
download hits 100%, then shows a spinner with no other feedback
until llama-server reports healthy. For a 130 GB model that spinner
freezes for five-plus minutes while the kernel pages shards into
the page cache. A new `GET /api/inference/load-progress` endpoint
samples `/proc/<pid>/status VmRSS` on the llama-server subprocess
against the sum of shard file sizes on disk, so the UI can render
a real bar plus rate / ETA during that window.
2. **Rate and ETA on downloads and loads.** Both the chat toast and
the training-start overlay used to show a static pair of numbers
(for example "15.4 of 140.8 GB"). A rolling 15-second window over
the existing byte-series now surfaces "85.3 MB/s, 24m 23s left"
beside that pair. The estimator is shared between the download
and load phases so the numbers don't reset when the phase flips.
Also fixes a pre-existing assignment bug uncovered while wiring this
up: `load_model` was storing the caller's `gguf_path` kwarg into
`self._gguf_path`, which is `None` on the HF-download code path. The
resolved on-disk path (`model_path`) is what llama-server actually
mmaps; downstream consumers need that. No existing reader used
`_gguf_path`, so this is a correctness fix for the new endpoint.
- Backend: `LlamaCppBackend.load_progress()`, `GET /api/inference/load-progress`, `LoadProgressResponse` Pydantic model.
- Frontend: `useTransferStats` hook, `formatRate` / `formatEta` helpers, `getLoadProgress` client, rewired chat toast and `DownloadRow` in the training overlay.
- Tests: `studio/backend/tests/test_llama_cpp_load_progress.py` covers empty states, mmap phase, ready phase, sharded total aggregation, missing gguf_path, and unreadable /proc (7 cases). `tsc -b` and `vite build` on the frontend both clean.
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