style(knowledge): solid blue-50 for the selected file card and row

The 7% brand-alpha tint let the white below bleed through, reading a
touch dimmer than the design's selection fill. blue-50 is the same hue
as an opaque swatch — still a brand token, so it keeps following the
blue⇄green theme.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/frontend/client/src/pages/knowledge/SpaceDetail/FileCard.tsx

@@ -549,7 +549,7 @@ export function FileCard({
                 !mobileListMode && "h-[160px]",
                 cardOpensPreviewOrFolder ? "cursor-pointer" : "cursor-default",
                 isSelected
-                    ? "bg-blue-500/[0.07]"
+                    ? "bg-blue-50"
                     : isNotParsed
                         ? "bg-[#fbfbfb]"
                         : "bg-white",

src/frontend/client/src/pages/knowledge/SpaceDetail/FileTable.tsx

@@ -893,7 +893,7 @@ function FileRow({
     const rowBg = isFolderDragOver
         ? "bg-blue-100 transition-colors duration-150"
         : isSelected
-            ? "bg-blue-500/[0.07] transition-colors duration-150 group-hover:bg-[#F8F8F8]"
+            ? "bg-blue-50 transition-colors duration-150 group-hover:bg-[#F8F8F8]"
             : "bg-white transition-colors duration-150 group-hover:bg-[#f7f7f7]";
     const {
         isRenaming,