feat(reader): send richer progress payload with chapter boundaries and zoom

The reader's saveProgress() now sends a more complete payload to the
backend so ProgressService has more data for enrichment and merge:

- chapter: computed from TOC boundary index instead of missing
- reading_mode: current display mode (page, chapter, percent, time-left)
- zoom_level: for fixed-layout books (renderer.zoomPercent / 100)
- current_page: real page number for fixed-layout, location.current for
  reflowable
- total_pages: section count for fixed-layout, location.total for
  reflowable

Adds computeChapterPageBoundaries(doc) for reflowable EPUBs that maps
TOC anchors to rendered page numbers, recomputes after fonts load.
Adds computeFixedLayoutChapterBoundaries() for fixed-layout books that
resolves TOC hrefs to page indices via view.resolveNavigation().

Updates reader.templ to expose isFixedLayout to Alpine init.

templates/reader.templ

@@ -154,7 +154,7 @@ templ ReaderChrome(user User, metadata ReaderMetadata, progress ReadingProgress)
 				<!-- Separator -->
 				<div class="w-px h-6 bg-gray-600 mx-1"></div>
 				<!-- Progress display -->
-				<div id="progress-display" x-text="progressText" @click="cycleProgressMode()" title="Click to change progress mode" class="text-sm min-w-[4rem] text-center cursor-pointer">
+				<div id="progress-display" x-text="progressText" @click="cycleProgressMode()" :title="progressTooltip()" class="text-sm min-w-[4rem] text-center cursor-pointer">
 					if progress.FormatGroup == "reflowable" {
 						if metadata.EstimatedPages > 0 {
 							{ fmt.Sprintf("%.0f%% · Page %d/%d", progress.Percentage, progress.CurrentPage, metadata.EstimatedPages) }