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

management/forms.py

@@ -881,7 +881,7 @@ class ClubSettingsForm(forms.ModelForm):
 
     class Meta:
         model = Club
-        fields = ["name", "legal_name", "contact_email", "logo", "primary_color", "secondary_color"]
+        fields = ["name", "legal_name", "contact_email", "website", "logo", "primary_color", "secondary_color"]
         widgets = {
             "primary_color": forms.TextInput(attrs={"placeholder": "#1e40af"}),
             "secondary_color": forms.TextInput(attrs={"placeholder": "#be185d"}),