Share compute stuck at "Starting…" — layer download hangs at ~70 MB

**Environment:** Buzz desktop v0.5.3 (aarch64), macOS 26.5.2, Apple M3, 6 GB AI memory

**Steps to reproduce:**
1. Settings -> Compute
2. 2. Model: `unsloth/gemma-4-E4B-it-GGUF:Q4_K_M` (the recommended one)
3. 3. Toggle "Share this machine"
**Expected:** Model downloads, sharing starts.

**Actual:** UI stays on "Starting…" indefinitely, then times out; the toggle becomes unresponsive until the app is restarted. After restart the status shows "Checking status…".

**Diagnosis from local cache inspection:** The base GGUF (4.98 GB) downloads successfully into `~/Library/Caches/huggingface/hub/models--unsloth--gemma-4-E4B-it-GGUF`. The subsequent meshllm layers download (`models--meshllm--gemma-4-E4B-it-Q4_K_M-layers`) stalls at exactly 70.8 MB and never resumes — confirmed across multiple attempts and app restarts, including a fresh reinstall to /Applications. Network is not the issue: a 5 GB browser download from Hugging Face completed in minutes on the same connection while Buzz was stuck.

Also noted: entering a local file path (plain or `file://`) in the Model field is rejected with "Expected an exact model ref", even though the UI text says a local file is accepted — so there is no user-side workaround.