feat(sync): serve web highlight colors to KOReader (mapped to its palette)

Reverses the earlier "no colors to the device" decision now that the
echo machinery makes it safe: GetMetadata maps the stored web hex to
KOReader's fixed color names (#ce93d8→purple, #90caf9→blue,
#a5d6a7→green, #ffd54f→yellow; pink maps to purple as the closest —
round-trip drift is prevented on the device by echo suppression, and
a device edit still wins). mapColorToKOReader restored for serving;
ingest (name→hex, preserve-on-echo) unchanged.

internal/handlers/koreader.go

@@ -993,10 +993,12 @@ func (h *KOReaderHandler) GetMetadata(c *echo.Context) error {
 				Text: ann.SelectionText,
 				Pos0: pos0,
 				Pos1: pos1,
-				// No color served: devices render their own default and
-				// cannot round-trip web colors — the web color only changes
-				// when the highlight is edited on the device (push carries
-				// the device color, ingested with the name→hex map).
+				// Web colors flow to the device, mapped to KOReader's named
+				// palette. Round-trip safety: the device suppresses the color
+				// when echoing un-edited applied entries (a pink→purple
+				// palette mismatch must not rewrite the stored hex), and an
+				// actual device edit pushes its color, which wins.
+				Color:    mapColorToKOReader(ann.Color.String),
 				Datetime: ann.CreatedAt.Time.Format(time.RFC3339),
 				DedupKey: ann.DedupKey.String,
 			}
@@ -1196,6 +1198,25 @@ func mapColorFromKOReader(name string) string {
 	return "#ffd54f"
 }
 
+var koreaderColorFromHex = map[string]string{
+	"#ffd54f": "yellow",
+	"#a5d6a7": "green",
+	"#90caf9": "blue",
+	"#ce93d8": "purple",
+	"#f48fb1": "purple",
+}
+
+// mapColorToKOReader normalizes a web hex swatch to the nearest KOReader
+// color name (default yellow) when serving to devices. Pink maps to purple
+// (the palette's closest); round-trip drift is prevented on the device by
+// suppressing echo colors for un-edited applied entries.
+func mapColorToKOReader(hex string) string {
+	if name, ok := koreaderColorFromHex[strings.ToLower(strings.TrimSpace(hex))]; ok {
+		return name
+	}
+	return "yellow"
+}
+
 //  1. A device-native CRE xpointer ("/body/...") in startPosition wins —
 //     round-trip identical for KOReader-pushed annotations (converting the
 //     stored CFI instead could drift and duplicate on the device).