feat(A.1 #1): surface active compute device + runtime-warning banner

Extends get_runtime_capabilities() with three new fields:
  - activeComputeDevice: {kind, label, reason} — "GPU (Vulkan)" on
    the happy path, "CPU (fallback)" with a reason when the Vulkan
    loader is absent at runtime. libloading::Library::new probes
    libvulkan.so.1 / vulkan-1.dll / libMoltenVK.dylib per target OS.
  - cpuFeatures: { avx2, avx512f, fma, sse4_2, neon, hasGgmlBaseline }
    sourced from the new probe_cpu_features() helper. hasGgmlBaseline
    is the one flag the Settings banner actually reads.
  - parallelModeAvailable: placeholder false until Phase A.4 lands
    the real GPU VRAM probe + GpuGuard semaphore permit logic.

Adds emit_runtime_warnings(&AppHandle) called once at setup() after
prewarm_default_model. Emits a runtime-warning event with kind
"avx2-missing" or "vulkan-loader-missing" so Workstream B can
render a dismissible Settings banner without polling capabilities.

Contract matches docs/whisper-ecosystem/workstream-A.md §Item #1 for
Workstream B to consume without waiting for Phase A.2 to land the
real whisper_print_system_info bridge.

Co-authored-by: jars <jakejars@users.noreply.github.com>

src-tauri/src/lib.rs

@@ -229,6 +229,11 @@ pub fn run() {
                 crate::commands::models::prewarm_default_model(whisper);
             }
 
+            // Runtime-warning banner: push CPU-feature + Vulkan-loader
+            // fallbacks to the frontend so Settings can render a one-line
+            // hint. No-ops on a fully-supported box.
+            crate::commands::models::emit_runtime_warnings(&app.handle());
+
             if let Err(e) = tray::setup(app) {
                 eprintln!("Failed to setup tray: {e}");
             }