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  matching.go MatchingHandler twins), superseded helpers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.

templates/utils.go

@@ -24,15 +24,6 @@ func activeClass(current, target string) string {
 	return base + "text-content-muted hover:bg-surface-hover hover:text-content"
 }
 
-// navItemClass returns classes for top-nav links, marking the active route.
-func navItemClass(current, target string) string {
-	base := "text-sm font-medium px-3 py-2 rounded-lg transition-colors "
-	if current == target {
-		return base + "text-content bg-surface-hover"
-	}
-	return base + "text-content-muted hover:text-content hover:bg-surface-hover"
-}
-
 // docsNavClass returns classes for docs sidebar links (server-rendered active
 // state — there is no client-side highlighter). indent sets the left padding,
 // deeper for subcategory group items.
@@ -322,30 +313,6 @@ func getDomainName(rawURL string) string {
 	return strings.TrimPrefix(hostname, "www.")
 }
 
-// formatAlternateInfo formats alternate series info as readable string
-func formatAlternateInfo(data []byte) string {
-	if len(data) == 0 {
-		return ""
-	}
-	var result map[string]interface{}
-	if err := json.Unmarshal(data, &result); err != nil {
-		return ""
-	}
-
-	var parts []string
-	if series, ok := result["alternate_series"].(string); ok && series != "" {
-		parts = append(parts, series)
-	}
-	if num, ok := result["alternate_number"].(float64); ok && num > 0 {
-		parts = append(parts, fmt.Sprintf("#%d", int(num)))
-	}
-	if count, ok := result["alternate_count"].(float64); ok && count > 0 {
-		parts = append(parts, fmt.Sprintf("(of %d)", int(count)))
-	}
-
-	return strings.Join(parts, " ")
-}
-
 // FormatInTimezone formats a time.Time in the specified timezone as MM-DD-YYYY HH:MM
 func FormatInTimezone(t time.Time, timezone string) string {
 	if t.IsZero() {
@@ -375,10 +342,6 @@ func textToString(t pgtype.Text) string {
 	return ""
 }
 
-func tagSliceToString(tags []string) string {
-	return strings.Join(tags, ", ")
-}
-
 func stringSliceToString(s []string) string {
 	return strings.Join(s, ", ")
 }