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  matching.go MatchingHandler twins), superseded helpers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/handlers/system_settings.go

@@ -184,29 +184,6 @@ type ScanSettingsResponse struct {
 	Message                 string `json:"message,omitempty"`
 }
 
-type UpdateTimezoneSettingsRequest struct {
-	DefaultTimezone string `json:"default_timezone" validate:"required"`
-}
-
-func (h *SystemSettingsHandler) UpdateTimezoneSettings(c *echo.Context) error {
-	var req UpdateTimezoneSettingsRequest
-	if err := c.Bind(&req); err != nil {
-		return c.JSON(http.StatusBadRequest, map[string]string{"error": "Invalid request"})
-	}
-	if _, err := time.LoadLocation(req.DefaultTimezone); err != nil {
-		return c.JSON(http.StatusBadRequest, map[string]string{"error": "Invalid timezone"})
-	}
-	err := h.db.UpdateSystemSetting(c.Request().Context(), database.UpdateSystemSettingParams{
-		SettingKey:   "default_timezone",
-		SettingValue: req.DefaultTimezone,
-	})
-	if err != nil {
-		return err
-	}
-	h.reload(c)
-	return c.JSON(http.StatusOK, map[string]string{"message": "Timezone updated"})
-}
-
 func (h *SystemSettingsHandler) UpdateScanSettings(c *echo.Context) error {
 	var req UpdateScanSettingsRequest
 	if err := c.Bind(&req); err != nil {