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>

backend/cmd/seed_e2e_user/main.go

@@ -0,0 +1,71 @@
+// seed_e2e_user creates or updates a deterministic test user for Playwright e2e.
+// Usage: go run ./backend/cmd/seed_e2e_user --db apps/kanban/operations.db
+//
+// Idempotent: safe to run repeatedly. The user "e2e_user" / password "e2e_test_pw_2026"
+// is intentional and used by apps/kanban/frontend/e2e/*.spec.ts when env vars are not set.
+package main
+
+import (
+	"database/sql"
+	"errors"
+	"flag"
+	"fmt"
+	"os"
+	"time"
+
+	_ "github.com/mattn/go-sqlite3"
+	"golang.org/x/crypto/bcrypt"
+)
+
+func main() {
+	dbPath := flag.String("db", "operations.db", "path to kanban operations.db")
+	username := flag.String("username", "e2e_user", "username")
+	password := flag.String("password", "e2e_test_pw_2026", "password")
+	displayName := flag.String("display", "E2E Test", "display name")
+	flag.Parse()
+
+	db, err := sql.Open("sqlite3", *dbPath)
+	if err != nil {
+		fail(err)
+	}
+	defer db.Close()
+
+	hash, err := bcrypt.GenerateFromPassword([]byte(*password), bcrypt.DefaultCost)
+	if err != nil {
+		fail(err)
+	}
+
+	now := time.Now().UTC().Format(time.RFC3339Nano)
+	id := "e2etest" + fmt.Sprintf("%x", time.Now().UnixNano())[:9]
+
+	// Try update first
+	res, err := db.Exec(
+		`UPDATE users SET password_hash=?, display_name=? WHERE username=?`,
+		string(hash), *displayName, *username,
+	)
+	if err != nil {
+		fail(err)
+	}
+	n, _ := res.RowsAffected()
+	if n > 0 {
+		fmt.Printf("updated existing user %q\n", *username)
+		return
+	}
+
+	_, err = db.Exec(
+		`INSERT INTO users (id, username, password_hash, display_name, created_at) VALUES (?, ?, ?, ?, ?)`,
+		id, *username, string(hash), *displayName, now,
+	)
+	if err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			fail(err)
+		}
+		fail(err)
+	}
+	fmt.Printf("created user %q (id=%s)\n", *username, id)
+}
+
+func fail(err error) {
+	fmt.Fprintln(os.Stderr, "seed_e2e_user:", err)
+	os.Exit(1)
+}