refactor: remove dead code across server, templ pages, and client modules

Deadcode-verified removals (RTA reachability from main, cross-checked
with -test and textual xrefs):

Go server:
- whole files: middleware/transaction.go, middleware/error_handler.go,
  middleware/password_validator.go (live registration machinery kept),
  handlers/sync.go, sync/offline.go
- unrouted endpoints: auth UpdateUsername/UpdateEmail, timezone
  settings handler, OPDS RegisterOPDS, Kobo LibrarySync handler,
  ReaderHandler.ShowReader, JSON-data twins on collections/library
- duplicate book-matching handlers (book_matching.go Handler twins,
  matching.go MatchingHandler twins), superseded helpers
  (router.parseUUID, handlers.ValidateDeviceToken, normalizePath,
  userExists, generateLockID)

Templ/client:
- never-routed pages: api_explorer.templ, queue.templ and their
  client modules api-explorer.ts, api-explorer-docs.ts, queue.ts
- fully orphaned wrapper modules events.ts, dom.ts
- broken clearSyncQueue (called a nonexistent endpoint) and the
  hidden, never-populated sync-queue section on the devices page
- 166+ unused exports demoted to private (knip-verified); ~85 truly
  dead declarations removed via tsc noUnusedLocals fixpoint
- stale 'interactive API Explorer' claims dropped from docs

The docs drift-gate package (internal/docs), templ-bound modules
(dashboard, stats, header, index, login, profile*, bookshelf,
collections, toast-error), sqlc-generated code, and web/static are
intentionally untouched.

internal/sync/cfi_converter.go

@@ -967,45 +967,6 @@ func findNodeAtCharOffset(root *html.Node, targetOffset int) (*html.Node, int) {
 	return found, currentOffset
 }
 
-func (c *CFIConverter) computePercentage(xp *CREXPointer, target *html.Node, remainingOffset int) float64 {
-	spine, err := c.loadSpine()
-	if err != nil {
-		return -1
-	}
-
-	charOffset := 0
-	for i := 1; i < xp.FragmentIndex; i++ {
-		doc, _, err := c.getContentDoc(i)
-		if err != nil {
-			continue
-		}
-		body := findBody(doc)
-		if body != nil {
-			charOffset += countTextChars(body)
-		}
-	}
-
-	charOffset += countTextCharsBefore(target)
-	charOffset += remainingOffset
-
-	totalChars := int64(0)
-	for i := range spine.items {
-		doc, _, err := c.getContentDoc(i + 1)
-		if err != nil {
-			continue
-		}
-		body := findBody(doc)
-		if body != nil {
-			totalChars += int64(countTextChars(body))
-		}
-	}
-
-	if totalChars <= 0 {
-		return -1
-	}
-	return float64(charOffset) / float64(totalChars)
-}
-
 type indexedNode struct {
 	node      *html.Node
 	textChunk []*html.Node
@@ -1157,7 +1118,6 @@ func findTextChunk(parent *html.Node, textNode *html.Node) (int, int, string) {
 	return -1, 0, ""
 }
 
-
 func findElementCFIIndex(parent *html.Node, element *html.Node) int {
 	indexed := indexChildNodes(parent)
 	for i, node := range indexed {