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/services/api.js

@@ -213,6 +213,21 @@ class ApiService {
         }
     }
 
+    /**
+     * Fetch GPU status: hardware, feature flags, and selected models per operation.
+     * Only meaningful when AI_PROVIDER=local_gpu.
+     * @returns {Promise<Object|null>}
+     */
+    async getGpuStatus() {
+        try {
+            const response = await fetch(`${this.baseUrl}/api/gpu/status`);
+            if (!response.ok) return null;
+            return response.json();
+        } catch {
+            return null;
+        }
+    }
+
     /**
      * Health check for the backend
      * @returns {Promise<boolean>}