feat(web): reusable flash/notification banner (PRG)

Add a one-shot flash component used across the app — connect success,
disconnect, account delete, and registration — instead of per-page ad-hoc
markup. setFlash queues a short-lived HttpOnly+SameSite cookie carrying an
opaque code; takeFlash consumes it on the next full-page render (not on
HTMX fragments). flashBanner maps the code to a styled, role=status banner;
the message text lives server-side in flashMessages so the cookie never
carries free text and a forged/unknown code renders nothing.

Wire it into the list page (the PRG landing spot for connect/registration)
and set it on registration and connect-callback success. Styled with the
existing design-system tokens; header gains an Account nav link.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/web/flash_internal_test.go

@@ -0,0 +1,54 @@
+package web
+
+import (
+	"context"
+	"strings"
+	"testing"
+)
+
+// TestFlashBannerRendersEachKind proves the reusable notification component
+// renders a banner with the right message and severity class for every flash
+// code, and renders nothing for an empty or unknown (e.g. forged) code.
+func TestFlashBannerRendersEachKind(t *testing.T) {
+	cases := []struct {
+		code     string
+		wantText string
+		wantKind string
+	}{
+		{flashConnected, "YouTube account connected", "flash-success"},
+		{flashConnectFailed, "Could not connect", "flash-error"},
+		{flashDisconnected, "Account disconnected", "flash-success"},
+		{flashDeleted, "account and all its data were deleted", "flash-success"},
+		{flashRegistered, "Welcome to Tapir", "flash-success"},
+	}
+	for _, tc := range cases {
+		t.Run(tc.code, func(t *testing.T) {
+			var sb strings.Builder
+			if err := flashBanner(tc.code).Render(context.Background(), &sb); err != nil {
+				t.Fatalf("render: %v", err)
+			}
+			got := sb.String()
+			if !strings.Contains(got, tc.wantText) {
+				t.Errorf("banner %q = %q, want it to contain %q", tc.code, got, tc.wantText)
+			}
+			if !strings.Contains(got, tc.wantKind) {
+				t.Errorf("banner %q = %q, want severity class %q", tc.code, got, tc.wantKind)
+			}
+			if !strings.Contains(got, `role="status"`) {
+				t.Errorf("banner %q must carry role=status for assistive tech, got %q", tc.code, got)
+			}
+		})
+	}
+}
+
+func TestFlashBannerRendersNothingForUnknownCode(t *testing.T) {
+	for _, code := range []string{"", "bogus", "<script>"} {
+		var sb strings.Builder
+		if err := flashBanner(code).Render(context.Background(), &sb); err != nil {
+			t.Fatalf("render: %v", err)
+		}
+		if got := strings.TrimSpace(sb.String()); got != "" {
+			t.Errorf("flashBanner(%q) = %q, want empty (no banner)", code, got)
+		}
+	}
+}