feat: make local GGUF setup resumable and restart-safe

[XYAIStudio]

FreeOS users can now start from a computer with neither Ollama nor an existing GGUF file. Settings → Models presents a pinned starter catalog, downloads the selected official Qwen GGUF with live progress, verifies its SHA-256, starts the bundled llama.cpp runtime, benchmarks it, and selects it for local chat only after a successful test.

This update closes the reliability gaps needed for the 0.0.7 flow:

• GGUF download jobs persist under the FreeOS home directory and resume from .part files after cancellation, network failure, or application restart.
• Concurrent downloads of the same catalog item are deduplicated and disk space is checked before transfer.
• The user-selected llama.cpp model is restored after FreeOS restarts; an explicit Stop clears that intent.
• Benchmark results persist across sessions, and interrupted downloads are surfaced with a one-click Resume action.
• API and Chinese installation documentation cover the catalog, download, llama.cpp, benchmark, and default-model flow.

The catalog currently offers pinned official Apache-2.0 Qwen2.5 1.5B and 3B Q4_K_M builds. The runtime remains pinned and checksum-verified, binds only to 127.0.0.1:11435, uses an argument vector rather than a shell command, logs under the FreeOS home directory, and stops only the process owned by FreeOS. Ollama remains an optional runtime and import path.

Validation:

• PYTHONUTF8=1 uv run pytest -n 4 -m "not live" — 3588 passed, 114 skipped
• uv run mypy src/octop — no issues in 590 source files
• uv run ruff check src tests and uv run ruff format --check src tests — 1158 files formatted
• focused backend tests — 13 passed
• focused dashboard tests — 10 passed
• dashboard ESLint — 0 errors; tsc --noEmit and production build passed
• GitHub Linux, Windows, and four-platform desktop package jobs rerun on each pushed commit

This PR remains stacked on #97 so the implementation and architecture plan can be reviewed separately. #97 now targets develop under the repository release policy.