Upload feature (f): demo scenarios (upload-slow/fail) + a11y (file-input label)

- upload-slow/upload-fail scenarios simulated in the adapter (XHR POST bypasses the
  HTTP interceptor); categories/status/delete already honour the global slow/error
- file-input gets a per-category accessible name (aria-label + visible button text)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/shared/infrastructure/scenario.ts

@@ -1,6 +1,15 @@
-export type Scenario = 'default' | 'slow' | 'loading' | 'empty' | 'error';
+export type Scenario =
+  | 'default'
+  | 'slow'
+  | 'loading'
+  | 'empty'
+  | 'error'
+  // upload-only (the multipart POST is hand-written XHR, so it bypasses the HTTP
+  // interceptor — these are simulated in upload.adapter.ts instead):
+  | 'upload-slow'
+  | 'upload-fail';
 
-const VALID: Scenario[] = ['default', 'slow', 'loading', 'empty', 'error'];
+const VALID: Scenario[] = ['default', 'slow', 'loading', 'empty', 'error', 'upload-slow', 'upload-fail'];
 
 /** Reads ?scenario= from the URL so a demo can force each async state. */
 export function currentScenario(): Scenario {