fix(upload): clear error and higher limit for large files

- raise upload limit to 100 MB
- return 413 file_too_large on MaxBytesError instead of misleading
  "expected multipart form"
- client-side per-file and total size pre-check with a clear message

backend/internal/photos/handlers.go

@@ -1,6 +1,7 @@
 package photos
 
 import (
+	"errors"
 	"io"
 	"net/http"
 	"strconv"
@@ -12,7 +13,7 @@ import (
 	"shared-album/internal/httpx"
 )
 
-const maxUploadSize = 50 << 20
+const maxUploadSize = 100 << 20
 
 type Handler struct {
 	svc *Service
@@ -43,7 +44,12 @@ func (h *Handler) Upload(c *gin.Context) {
 	c.Request.Body = http.MaxBytesReader(c.Writer, c.Request.Body, maxUploadSize)
 	form, err := c.MultipartForm()
 	if err != nil {
-		httpx.RespondError(c, httpx.BadRequest("invalid_request", "expected multipart form"))
+		var maxErr *http.MaxBytesError
+		if errors.As(err, &maxErr) {
+			httpx.RespondError(c, httpx.NewError(http.StatusRequestEntityTooLarge, "file_too_large", "files exceed the maximum upload size (100 MB)"))
+			return
+		}
+		httpx.RespondError(c, httpx.BadRequest("invalid_request", "invalid multipart form"))
 		return
 	}
 	files := form.File["files"]