fix(sync): let content-equal echoes refresh drifted locators

The LWW skip path compared only annotation content (text, color, note,
percentages) — locator columns were not part of 'changed'. A device echo
with identical content therefore resolved to skip, and the freshly
re-derived canonical CFIs were discarded in the same request that
computed them: a highlight whose stored start anchor had been corrupted
by the old percentage-exact bug could never heal, because every
subsequent echo carried the same text and was skipped before the
locator columns were written. Observed live: a push converted the
cross-block highlight's anchors correctly (structural, heading to
paragraph) yet the row kept its garbage start CFI and the highlight
stayed unpaintable on the web.

Echo saves now treat a non-empty incoming locator that differs from the
stored one as a change (locatorRefresher): empty locators still coalesce
(no drift), and once healed the echo produces identical CFIs, so the
steady state remains skip — no write churn. Highlights check
epubcfi_start/end; bookmarks check cfi_position/position.

applyBookmarkLWW also gains the empty-locator coalescing the highlight
path already had: web bookmark edits carry no device locators, and a
title/note edit must not wipe the stored device-native position.

internal/sync/annotations.go

@@ -279,7 +279,9 @@ func (s *AnnotationService) compareIncoming(req SaveHighlightRequest, existing d
 			textEq(req.Color, existing.Color) &&
 			textEq(req.NoteText, existing.NoteText) &&
 			floatEq(req.PercentageStart, existing.PercentageStart) &&
-			floatEq(req.PercentageEnd, existing.PercentageEnd)
+			floatEq(req.PercentageEnd, existing.PercentageEnd) &&
+			locatorRefresher(req.EpubcfiStart, existing.EpubcfiStart) &&
+			locatorRefresher(req.EpubcfiEnd, existing.EpubcfiEnd)
 		return !contentSame, !contentSame
 	}
 
@@ -290,6 +292,22 @@ func (s *AnnotationService) compareIncoming(req SaveHighlightRequest, existing d
 	return req.ModifiedAt.After(existingMod.Time), true
 }
 
+// locatorRefresher reports whether an incoming locator leaves the stored
+// one untouched: either the push carried none (empty coalesces to the
+// stored value in applyLWW) or it matches what is stored. A non-empty
+// incoming locator that DIFFERS is a deliberate refresh: device echoes
+// re-derive their canonical CFIs on every push, and an improvement (a
+// converter fix re-landing a corrupted anchor, drift repair) must reach
+// the row even when the annotation content is otherwise identical —
+// without this, content-equal echoes resolve to "skip" and a bad stored
+// locator can never heal.
+func locatorRefresher(incoming string, existing pgtype.Text) bool {
+	if incoming == "" {
+		return true
+	}
+	return textEq(incoming, existing)
+}
+
 func (s *AnnotationService) TombstoneHighlight(
 	ctx context.Context,
 	userID, mediaItemID pgtype.UUID,
@@ -744,13 +762,25 @@ func (s *AnnotationService) applyBookmarkLWW(ctx context.Context, req SaveBookma
 	}
 	deviceData := mergeDeviceSyncData(existing.DeviceSyncData, req.Source, req.DeviceSyncData)
 
+	// Web saves carry no device locators: keep the stored ones so a web
+	// title/note edit never wipes the device-native position (mirrors the
+	// highlight path's coalescing).
+	cfiPosition := req.CFIPosition
+	if cfiPosition == "" {
+		cfiPosition = existing.CfiPosition.String
+	}
+	position := req.Position
+	if position == "" {
+		position = existing.Position.String
+	}
+
 	bm, err := s.db.UpdateMediaBookmarkForSync(ctx, database.UpdateMediaBookmarkForSyncParams{
 		ID:                 existing.ID,
 		PageNumber:         pgInt4(req.PageNumber),
 		ChapterNumber:      pgInt4(req.ChapterNumber),
-		CfiPosition:        pgText(req.CFIPosition),
+		CfiPosition:        pgText(cfiPosition),
 		Title:              req.Title,
-		Position:           pgText(req.Position),
+		Position:           pgText(position),
 		Notes:              pgText(req.Notes),
 		PercentageLocation: pgFloat8(req.PercentageLoc),
 		EpubcfiLocation:    pgText(req.EpubcfiLocation),
@@ -775,7 +805,9 @@ func (s *AnnotationService) applyBookmarkLWW(ctx context.Context, req SaveBookma
 func (s *AnnotationService) compareIncomingBookmark(req SaveBookmarkRequest, existing database.MediaBookmarks) (incomingNewer bool, contentChanged bool) {
 	if req.ModifiedAt.IsZero() {
 		contentSame := strings.EqualFold(req.Title, existing.Title) &&
-			textEq(req.Notes, existing.Notes)
+			textEq(req.Notes, existing.Notes) &&
+			locatorRefresher(req.CFIPosition, existing.CfiPosition) &&
+			locatorRefresher(req.Position, existing.Position)
 		return !contentSame, !contentSame
 	}
 	existingMod := existing.LastModifiedAt