refactor(reader): one comic-likeness predicate — panel detection was a byte-identical twin

shouldEnablePanelDetection and ShouldTreatAsComic had identical truth
tables (manga/comics library x fixed_layout/comic_archive) under
different names — a drift trap: adding a library type to one silently
missed the other. One predicate now gates both features, with the
re-split instruction in its doc comment should the features ever
legitimately diverge.

internal/handlers/reader.go

@@ -102,7 +102,9 @@ func (h *ReaderHandler) ShowReader(c *echo.Context) error {
 	}
 
 	// Determine if panel detection should be enabled
-	enablePanelDetection := shouldEnablePanelDetection(libraryWithType.TypeName, mediaItem.FormatGroup)
+	// Same comic-likeness predicate as treatAsComic below — one function
+	// on purpose (see ShouldTreatAsComic's doc).
+	enablePanelDetection := ShouldTreatAsComic(libraryWithType.TypeName, mediaItem.FormatGroup)
 	treatAsComic := ShouldTreatAsComic(libraryWithType.TypeName, mediaItem.FormatGroup)
 
 	// Get reading progress
@@ -639,6 +641,12 @@ func (h *ReaderHandler) generatePDFThumbnail(fullPath string, page int, width, h
 // only — no text-selection highlights, no annotations, no in-book search.
 // Comic archives already behave this way; this flag extends it to fixed-
 // layout PDFs shelved in comic libraries (scanned or official manga PDFs).
+//
+// This is ALSO the panel-detection gate (reader.go's enablePanelDetection):
+// the two checks were once separate functions with byte-identical bodies —
+// a drift trap (adding a library type to one silently missed the other).
+// If comic treatment and panel detection ever need to diverge, re-split
+// them with a comment saying why.
 func ShouldTreatAsComic(libraryType string, formatGroup string) bool {
 	comicLibraries := map[string]bool{
 		"manga":  true,
@@ -652,19 +660,3 @@ func ShouldTreatAsComic(libraryType string, formatGroup string) bool {
 
 	return comicLibraries[libraryType] && fixedFormats[formatGroup]
 }
-
-// shouldEnablePanelDetection determines if panel detection should be enabled
-// based on library type and format group
-func shouldEnablePanelDetection(libraryType string, formatGroup string) bool {
-	panelLibraries := map[string]bool{
-		"manga":  true,
-		"comics": true,
-	}
-
-	panelFormats := map[string]bool{
-		"fixed_layout":  true,
-		"comic_archive": true,
-	}
-
-	return panelLibraries[libraryType] && panelFormats[formatGroup]
-}