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/cfi_converter.go

@@ -626,7 +626,12 @@ func flattenDocumentForSearch(body *html.Node) ([]rune, []docRunePos) {
 		lastBlock = block
 		off := 0
 		for _, r := range n.Data {
-			if unicode.IsSpace(r) {
+			// Zero-width format characters are line-break opportunities,
+			// not word characters: devices render them as breaks and
+			// capture plain spaces, so they must not fuse words in the
+			// search stream. ZWJ (U+200D) is exempt — it glues emoji
+			// sequences.
+			if unicode.IsSpace(r) || r == '\u200b' || r == '\u200c' || r == '\ufeff' {
 				pendingSpace = true
 			} else {
 				if pendingSpace && len(runes) > 0 {
@@ -644,9 +649,12 @@ func flattenDocumentForSearch(body *html.Node) ([]rune, []docRunePos) {
 	var walk func(n *html.Node)
 	walk = func(n *html.Node) {
 		if n.Type == html.TextNode {
-			if strings.TrimSpace(n.Data) != "" {
-				appendNode(n)
-			}
+			// Whitespace-only nodes still participate: a space carried in
+			// its own inline span (<span class="sp"> </span>) is a real
+			// word separator, and skipping it fuses adjacent words in the
+			// search stream. appendNode turns such nodes into pending
+			// space without emitting anything.
+			appendNode(n)
 			return
 		}
 		for c := n.FirstChild; c != nil; c = c.NextSibling {