feat(llm B.1 #27): Test LLM button with classified error diagnostics

The brief's pain point is opaque load failures: llama-cpp-2's errors
bubble up as raw C++ strings ("cudaMalloc failed: out of memory",
"invalid gguf magic"). A user seeing that has no path to recovery.

New backend command test_llm_model runs a staged diagnostic:
  1. Model not downloaded → `not-downloaded` + download hint.
  2. File size ≤90% of expected → `incomplete` (stalled download)
     + re-download hint. Matters because llama-cpp-2 can segfault
     on truncated GGUF rather than returning cleanly.
  3. Requested model already loaded → `ready`, no side effects.
  4. Otherwise attempt a real load. On failure, classify_llm_load_error
     maps the raw string to one of:
       - load-failed-vram         (OOM / cudaMalloc / allocation)
       - load-failed-corrupt      (GGUF magic / unsupported format)
       - load-failed-permission   (permission denied / access denied)
       - load-failed-other        (catch-all)
     Each category has a prewritten actionable hint pointing at the
     specific Settings surface (tier picker, re-download, file perms).

classify_llm_load_error is pure-string and unit-tested — 8 cases
covering the main categories plus edge cases (OOM alias, Windows
"Access is denied", unknown errors). Ordered narrow-to-broad so
overlap doesn't misclassify.

Settings UI gets a "Test" button in the AI section's action row,
visible whenever the model is downloaded (both downloaded-idle and
loaded states). Shows inline hint below the status line when the
test surfaces one. Refreshes both local and global LLM status after
the test since a successful test implicitly loads the model.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -258,6 +258,7 @@ pub fn run() {
             commands::llm::unload_llm_model,
             commands::llm::delete_llm_model,
             commands::llm::get_llm_status,
+            commands::llm::test_llm_model,
             commands::llm::cleanup_transcript_text_cmd,
             // Parakeet model management
             commands::models::download_parakeet_model,