feat(kanban): requester input empty + keyboard nav (issue 0088)

CardForm: drop pre-fill of requester from logged user; Enter inside the
Autocomplete no longer submits the form (Mantine handles dropdown
selection; arrows + Enter pick option without closing modal). Submit
remains via "Crear" button or Ctrl+Enter from description.

Adds data-field="requester" and data-test="add-card" selectors for stable
e2e queries.

Tests:
- vitest component test (CardForm.test.tsx): empty input, Enter does not
  submit, submit only via button. Dropdown arrow nav covered by e2e
  (jsdom portal handling is brittle).
- Playwright e2e (requester-input.spec.ts) using new browser capability
  group (pw_kanban_login, pw_keyboard_sequence) from registry.
- seed_e2e_user CLI to create deterministic test user against
  operations.db (bcrypt via standard backend hash).

Setup additions (frontend/):
- vitest + @testing-library + jsdom devDeps
- @playwright/test devDep + playwright.config.ts
- src/test/setup.ts polyfills jsdom for Mantine (matchMedia,
  visualViewport, document.fonts, ResizeObserver)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/App.tsx

@@ -545,7 +545,7 @@ export function App() {
           users={users}
           requesterOptions={requesterOptions}
           tagOptions={tagOptions}
-          initial={{ requester: auth.user?.display_name || auth.user?.username || "" }}
+          initial={{ requester: "" }}
           submitLabel="Crear"
           onCancel={() => modals.close(id)}
           onSubmit={async (v) => {