fix(marketing): button macro safety + placeholder assets + mobile login

- Button macro: variants/sizes use .get() with primary/md fallback (no KeyError),
  add as_button=True parameter for form submit CTAs (forms in B-2.2 signup,
  B-2.7 checkout)
- Header: drop hidden sm:inline-block on Connexion link so login is always
  visible from mobile (avoids UX dead-end before A-2.7 hamburger lands)
- Add placeholder static/images/favicon.svg (brand gradient + 'D')
- Add placeholder 1x1 PNG static/images/og/og-default.png (replaced in A-3.3
  with real 1200x630 branded OG image)

static/images/favicon.svg

@@ -0,0 +1,11 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64">
+  <defs>
+    <linearGradient id="g" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0%" stop-color="#0062ff"/>
+      <stop offset="52%" stop-color="#00bdd8"/>
+      <stop offset="100%" stop-color="#00c896"/>
+    </linearGradient>
+  </defs>
+  <rect width="64" height="64" rx="14" fill="url(#g)"/>
+  <text x="32" y="46" text-anchor="middle" font-family="Inter, system-ui, sans-serif" font-weight="900" font-size="40" fill="#fff">D</text>
+</svg>