Add Club.website; rebuild Colours per D9's exact markup; add a mobile app preview

- club.website (URLField, blank) editable from both the club's own Identity
  page and the platform control panel's club form.
- Colours card rebuilt from the design canvas's literal D9 markup rather
  than the prior in-between version: swatch + mono hex merged into one
  bordered row (the swatch input and hex text input are the same real form
  field, just laid out together), and a contrast-check row of two colour-
  filled boxes instead of a plain text list -- labelled dynamically
  ("Black"/"White" on primary/secondary) since a club can pick a colour
  light enough that black, not white, is the real computed text colour.
- Logo card restyled to D9's dashed drop-zone look.
- Live preview gains a mobile app mock (marked "Coming soon") alongside the
  existing sidebar mock, using the same primary/secondary colours, ahead of
  that surface actually being built.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ECGMEwrc2k4D8VQuwjstj9

club/migrations/0027_club_website.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.6 on 2026-08-20 07:08
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('club', '0026_memberrequirementstatus_is_bypassed_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='club',
+            name='website',
+            field=models.URLField(blank=True, help_text="The club's own site, if it has one -- shown alongside its RosterChief pages, not used for anything else yet.", verbose_name='website'),
+        ),
+    ]