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  matching.go MatchingHandler twins), superseded helpers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/router/helpers.go

@@ -92,10 +92,6 @@ func pingDB(cfg *Config, ctx context.Context) error {
 	return nil
 }
 
-func parseUUID(s string) (uuid.UUID, error) {
-	return uuid.Parse(s)
-}
-
 func uuidToPGType(u uuid.UUID) pgtype.UUID {
 	return pgtype.UUID{Bytes: u, Valid: true}
 }

internal/router/router.go

@@ -237,14 +237,6 @@ func createJWTMiddleware(cfg *Config) echo.MiddlewareFunc {
 	})
 }
 
-// userExists checks the users table for the token's claimed user. Runs on
-// every authenticated request: a primary-key lookup, cheap at this scale,
-// and it makes deletion revocable the moment it happens.
-func userExists(cfg *Config, id uuid.UUID) bool {
-	_, err := cfg.Queries.GetUser(context.Background(), uuidToPGType(id))
-	return err == nil
-}
-
 // fetchLiveUser loads the full user row for the JWT stub — the ghost-session
 // guard and the timezone travel together on this one per-request lookup.
 func fetchLiveUser(cfg *Config, id uuid.UUID) (database.GetUserRow, bool) {