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  matching.go MatchingHandler twins), superseded helpers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/services/media_scanner_comic_test.go

@@ -6,7 +6,6 @@ import (
 	"bytes"
 	"compress/gzip"
 	"encoding/xml"
-	"fmt"
 	"image"
 	"image/color"
 	"image/png"
@@ -595,21 +594,6 @@ func createTestTarGz(path string) error {
 	return nil
 }
 
-// Example usage
-func Example_extractComicMetadata() {
-	// Extract metadata from a comic archive
-	comicInfo, coverImage, err := extractComicMetadata("/path/to/comic.cbz")
-	if err != nil {
-		fmt.Printf("Error: %v\n", err)
-		return
-	}
-
-	fmt.Printf("Title: %s\n", comicInfo.Title)
-	fmt.Printf("Series: %s\n", comicInfo.Series)
-	fmt.Printf("Issue: %d\n", comicInfo.Number)
-	fmt.Printf("Cover size: %d bytes\n", len(coverImage))
-}
-
 func TestNormalizeMangaType(t *testing.T) {
 	tests := []struct {
 		name     string