fix: sidecar response interceptor + folder proxy

- Added 401-handling response interceptor to sidecar axios instance
  (matches existing http instance) so expired/invalid tokens redirect
  to login instead of showing raw 404/401 errors
- Added GET /api/sidecar/folders — proxies PhotoPrism's
  /api/v1/folders/originals with BasePath post-filter
- Updated listFolders() frontend to call sidecar proxy
- Updated plan with remaining fixes

sidecar/handlers_folders_proxy.go

@@ -0,0 +1,70 @@
+package main
+
+import (
+	"encoding/json"
+	"net/http"
+	"strings"
+
+	"github.com/gin-gonic/gin"
+)
+
+// handleFoldersProxy proxies PhotoPrism's /api/v1/folders/originals and
+// post-filters by the caller's BasePath so the folder tree only shows
+// folders under the user's library root.
+//
+// Route: GET /api/sidecar/folders  (behind requireSession)
+func handleFoldersProxy(pp *ppClient) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		token := ctxToken(c)
+		basePath := ctxBasePath(c)
+
+		// Forward query params to PhotoPrism.
+		query := c.Request.URL.RawQuery
+		if query == "" {
+			query = "recursive=true&uncached=true&files=false"
+		}
+
+		resp, err := pp.call(c.Request.Context(), http.MethodGet, "/api/v1/folders/originals?"+query, token, nil)
+		if err != nil || !resp.OK {
+			c.JSON(http.StatusBadGateway, gin.H{"error": "upstream folders request failed"})
+			return
+		}
+
+		// Decode the response — PhotoPrism returns { folders: [...] }.
+		var payload struct {
+			Folders []map[string]any `json:"folders"`
+		}
+		if err := json.Unmarshal(resp.Body, &payload); err != nil {
+			c.Data(resp.Status, "application/json", resp.Body)
+			return
+		}
+
+		// If the user has no BasePath (admin/empty), return as-is.
+		if basePath == "" {
+			c.JSON(http.StatusOK, payload)
+			return
+		}
+
+		prefix := basePath + "/"
+
+		// Post-filter folders by Path field only — frontend handles BasePath
+		// prefix stripping via toUserPath().
+		filtered := make([]map[string]any, 0, len(payload.Folders))
+		for _, f := range payload.Folders {
+			rawPath, ok := f["Path"]
+			if !ok {
+				continue
+			}
+			pathStr, ok := rawPath.(string)
+			if !ok {
+				continue
+			}
+			// Keep only folders under the user's base path.
+			if pathStr == basePath || strings.HasPrefix(pathStr, prefix) {
+				filtered = append(filtered, f)
+			}
+		}
+
+		c.JSON(http.StatusOK, gin.H{"folders": filtered})
+	}
+}