feat(handlers): add file icons with standard and retro variants

Introduce file icon support to the file browser. Icons are loaded on
startup and mapped based on file name and content type.

- Load file icon mappings in the App handler initialization.
- Add `HasThumbnail`, `IconURL`, and `IconRetroURL` to the file view.
- Update CSS to support displaying file icons alongside thumbnails.
- Add retro theme support to swap standard icons with pixelated retro
  variants when the retro theme is active.

backend/libs/handlers/icons_test.go

@@ -0,0 +1,54 @@
+package handlers
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+func TestFileIconMapLoadsAndResolvesCommonTypes(t *testing.T) {
+	icons, err := loadFileIcons(filepath.Join("..", "..", "static"))
+	if err != nil {
+		t.Fatalf("loadFileIcons returned error: %v", err)
+	}
+
+	tests := []struct {
+		name         string
+		contentType  string
+		wantStandard string
+		wantRetro    string
+	}{
+		{
+			name:         "photo.jpg",
+			contentType:  "application/octet-stream",
+			wantStandard: "image-document-svgrepo-com.svg",
+			wantRetro:    "shimgvw.dll_14_1-2.png",
+		},
+		{
+			name:         "movie.mkv",
+			contentType:  "",
+			wantStandard: "video-document-svgrepo-com.svg",
+			wantRetro:    "wmploc.dll_14_504-2.png",
+		},
+		{
+			name:         "archive.7z",
+			contentType:  "",
+			wantStandard: "zip-document-svgrepo-com.svg",
+			wantRetro:    "zipfldr.dll_14_101-2.png",
+		},
+		{
+			name:         "unknown.bin",
+			contentType:  "application/octet-stream",
+			wantStandard: "txt-document-svgrepo-com.svg",
+			wantRetro:    "shell32.dll_14_152-2.png",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := icons.lookup(tt.name, tt.contentType)
+			if got.Standard != tt.wantStandard || got.Retro != tt.wantRetro {
+				t.Fatalf("lookup returned %+v, want standard=%q retro=%q", got, tt.wantStandard, tt.wantRetro)
+			}
+		})
+	}
+}