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

controlpanel/templates/controlpanel/club_form.html

@@ -27,6 +27,7 @@
                     {% form_field form.name %}
                     {% form_field form.legal_name %}
                     {% form_field form.contact_email %}
+                    {% form_field form.website %}
                     {% form_field form.slug %}
                     {% form_field form.sport_type %}
                 </div>