feat: GPU capability display in UI + GTX 1060 6GB SDXL fix

Model selection:
- Add sdxl_offload tier (eff_vram ≥ 4.0 GB) for GTX 1060 6GB and Quadro
  6GB cards that were falling through to SD 2.1 despite SDXL fitting with
  model_cpu_offload. Cards with 5.3 GB effective VRAM now get SDXL quality.
- Update _tier_label(), _caps(), _build_warnings() for new tier.

Frontend GPU display:
- api.js: add getGpuStatus() fetching /api/gpu/status
- capabilities.js: add getGpuStatus() export with own LRU cache;
  refreshCapabilities() now also resets GPU status cache
- provider-badge.js: when AI_PROVIDER=local_gpu show green badge with
  GPU name, tier, VRAM, CC, feature flags, and capabilities in tooltip.
  Strip "NVIDIA GeForce" prefix so "GTX 1060 6GB" fits in badge.
- ai_provider_settings.js: add local_gpu to all provider dropdowns;
  show GPU info panel (device, VRAM, CC, features, tier, model table per
  operation) in the settings dialog when a GPU is detected.

https://claude.ai/code/session_01WVDg7amsy1TTtxvpku7bcM

frontend/src/js/api/capabilities.js

@@ -19,6 +19,8 @@ const DEFAULT_CAPS = {
 
 let _caps = null;
 let _fetchPromise = null;
+let _gpuStatus = null;
+let _gpuFetchPromise = null;
 
 /**
  * Return capabilities (fetched lazily, cached thereafter).
@@ -48,12 +50,29 @@ export function hasRemote() {
     return !!(_caps?.remote?.healthy);
 }
 
+/**
+ * Fetch and cache detailed GPU status (hardware, feature flags, model selection per op).
+ * Calls /api/gpu/status — only meaningful when AI_PROVIDER=local_gpu.
+ * Returns null on error.
+ */
+export async function getGpuStatus() {
+    if (_gpuStatus !== null) return _gpuStatus;
+    if (!_gpuFetchPromise) {
+        _gpuFetchPromise = apiService.getGpuStatus()
+            .then(data => { _gpuStatus = data; return _gpuStatus; })
+            .catch(() => { _gpuStatus = null; return null; });
+    }
+    return _gpuFetchPromise;
+}
+
 /**
  * Invalidate cache and re-fetch (call after saving provider settings).
  */
 export async function refreshCapabilities() {
     _caps = null;
     _fetchPromise = null;
+    _gpuStatus = null;
+    _gpuFetchPromise = null;
     return getCapabilities();
 }
 
@@ -62,4 +81,4 @@ export async function refreshCapabilities() {
  */
 getCapabilities();
 
-export default { getCapabilities, getCachedCapabilities, hasRemote, refreshCapabilities };
+export default { getCapabilities, getCachedCapabilities, hasRemote, refreshCapabilities, getGpuStatus };