mobile: off-canvas sidebar, responsive grids/charts/top-bar

- Sidebar becomes slide-in overlay under 768px with backdrop
- Hamburger button in top-bar for mobile nav toggle
- Grids stack single-column, charts shrink to 280px
- Tables scroll horizontally with touch momentum
- Status text hidden on mobile (dot stays visible)
- Card headers/actions stack vertically
- Period selectors scroll horizontally

also: image size validation clamp in handleImageGenerations

internal/server/server.go

@@ -850,6 +850,31 @@ func (s *Server) handleImageGenerations(c *gin.Context) {
 		}
 	}
 
+	// ponytail: per-model valid size sets. Add new models here.
+	if req.Size != nil {
+		validSizes := map[string][]string{
+			"gpt-image": {"1024x1024", "1024x1536", "1536x1024", "auto"},
+			"dall-e-3":  {"1024x1024", "1024x1792", "1792x1024", "auto"},
+			"dall-e-2":  {"256x256", "512x512", "1024x1024", "auto"},
+		}
+		for prefix, sizes := range validSizes {
+			if strings.HasPrefix(req.Model, prefix) {
+				valid := false
+				for _, s := range sizes {
+					if *req.Size == s {
+						valid = true
+						break
+					}
+				}
+				if !valid {
+					*req.Size = "1024x1024"
+					log.Printf("[WARN] Unsupported size for %s, clamped to 1024x1024", req.Model)
+				}
+				break
+			}
+		}
+	}
+
 	provider, ok := s.providers[providerName]
 	if !ok {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": fmt.Sprintf("Provider %s not enabled or supported", providerName)})