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/models.py

@@ -48,6 +48,7 @@ class Club(UUIDModel):
         blank=True,
         help_text=_("The club's public address, shown to people the club writes to or asks to get in touch -- e.g. a parent claiming a child. Falls back to nothing being shown at all, so it's worth setting."),
     )
+    website = models.URLField(_("website"), blank=True, help_text=_("The club's own site, if it has one -- shown alongside its RosterChief pages, not used for anything else yet."))
 
     logo = models.FileField(
         _("logo"),