sidecar: scoped labels + counts proxy (fixes cross-user label leak)

- New GET /api/sidecar/labels — proxies PP's labels, recalculates
  PhotoCount per user's BasePath via DB query
- New GET /api/sidecar/counts — returns user-scoped sidebar badges
  (all, review, archived, private, photos, videos, favorites)
- Fixed auth middleware to expose userUID and basePath on context
- Fixed ppClient.resolveSession — uses correct endpoint
  (GET /api/v1/session, not /api/v1/session/{token}) and correct
  JSON field names (UID, Name instead of UserUID, UserName)
- Frontend: listLabels now calls /api/sidecar/labels instead of /api/v1/labels

sidecar/auth.go

@@ -26,6 +26,8 @@ func requireSession(pp *ppClient) gin.HandlerFunc {
 		}
 		c.Set("token", token)
 		c.Set("userName", user.UserName)
+		c.Set("userUID", user.UserUID)
+		c.Set("basePath", user.BasePath)
 		c.Next()
 	}
 }
@@ -57,3 +59,29 @@ func ctxUserName(c *gin.Context) string {
 	}
 	return s
 }
+
+// ctxUserUID returns the PhotoPrism user UID resolved by requireSession.
+func ctxUserUID(c *gin.Context) string {
+	v, ok := c.Get("userUID")
+	if !ok {
+		return ""
+	}
+	s, ok := v.(string)
+	if !ok {
+		return ""
+	}
+	return s
+}
+
+// ctxBasePath returns the PhotoPrism user BasePath resolved by requireSession.
+func ctxBasePath(c *gin.Context) string {
+	v, ok := c.Get("basePath")
+	if !ok {
+		return ""
+	}
+	s, ok := v.(string)
+	if !ok {
+		return ""
+	}
+	return s
+}