fix(sync): anchor device highlight ranges to selection text

KOReader highlights converted to wrong or unresolvable web CFIs
whenever a selection crossed text-node boundaries. Three compounding
defects: ParseCREXPointer discarded the text()[N] node index and
applied .offset as a global block offset; the end anchor converted
with an empty context (blind structural landing, zero verification);
and the block-containment check accepted wrong-node landings because
a selection's text always occurs somewhere in its own block. pos0 and
pos1 were converted independently, yielding mismatched start/end base
paths and backwards ranges - highlights stored but never painted, or
painted at the wrong location, in the web reader.

Add ConvertHighlightRange: both endpoints are anchored by locating the
selection text in the flattened document text (whitespace-normalized,
crossing block boundaries), with the device locators demoted to
disambiguation hints. A single document match yields a same-spine,
ordered start/end pair by construction. Falls back to the legacy
per-endpoint path when the text cannot be located, so behavior is
never worse than before.

Also fix the shared flatten stream: zero-width format characters
(ZWSP/ZWNJ/BOM) and whitespace-only inline spans (<span> </span>)
now act as word separators instead of fusing adjacent words, which
prevented text matching in real books.

Verified against 28 real device highlights from production: converter
output now matches the text-search oracle on all of them (previously
8/28 correct, 18 with mismatched base paths, 2 backwards).

internal/sync/locators.go

@@ -125,6 +125,38 @@ func ConvertToCanonical(
 	}
 }
 
+// ConvertHighlightRangeToCanonical is the highlight-specific twin of
+// ConvertToCanonical: it anchors a device highlight's pos0/pos1 pair by the
+// selection text so both endpoints derive from one document match — same
+// spine, ordered — instead of two independent structural landings. Returns
+// a "fallback" locator with an empty CFI when the text cannot be located;
+// callers fall back to ConvertToCanonical per endpoint.
+func ConvertHighlightRangeToCanonical(
+	epubPath string,
+	formatGroup string,
+	pos0 string,
+	pos1 string,
+	contextText string,
+	percentage float64,
+) CanonicalLocator {
+	if !isConvertible(formatGroup) || epubPath == "" {
+		return CanonicalLocator{CFI: pos0, Precision: "passthrough", Percentage: percentage}
+	}
+	if !IsCREXPointer(pos0) {
+		return CanonicalLocator{CFI: pos0, Precision: "already-standard", Percentage: percentage}
+	}
+	result := cachedConverter(epubPath).ConvertHighlightRange(pos0, pos1, contextText, percentage)
+	if result == nil || result.EPUBCFI == "" {
+		return CanonicalLocator{CFI: "", Precision: "fallback", Percentage: percentage}
+	}
+	return CanonicalLocator{
+		CFI:        result.EPUBCFI,
+		EndCFI:     result.EndEPUBCFI,
+		Precision:  result.Precision,
+		Percentage: result.Percentage,
+	}
+}
+
 func ConvertFromCanonical(
 	source LocatorSource,
 	canonicalCFI string,