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)

templates/macros/button.html

@@ -1,15 +1,24 @@
-{# Reusable button macro — primary (gradient + glow), secondary (white outline), ghost (transparent on dark) #}
-{%- macro button(text, href='#', variant='primary', size='md', icon=None, target=None, rel=None) -%}
-{%- set classes = {
+{# Reusable button macro. Variants: primary | secondary | ghost (default: primary). Sizes: sm | md | lg (default: md). #}
+{%- macro button(text, href='#', variant='primary', size='md', icon=None, target=None, rel=None, as_button=False, type='button') -%}
+{%- set variants = {
   'primary': 'grad-bg shadow-cta hover:shadow-cta-hover hover:-translate-y-px',
   'secondary': 'bg-white text-brand-navy border border-brand-border hover:bg-brand-bg',
   'ghost': 'text-white border border-white/[0.08] hover:bg-white/[0.05]'
-}[variant] -%}
-{%- set sizing = {
+} -%}
+{%- set sizes = {
   'sm': 'px-3 py-1.5 text-sm',
   'md': 'px-5 py-2.5 text-[15px]',
   'lg': 'px-6 py-3 text-base'
-}[size] -%}
+} -%}
+{%- set classes = variants.get(variant, variants['primary']) -%}
+{%- set sizing = sizes.get(size, sizes['md']) -%}
+{%- if as_button -%}
+<button type="{{ type }}"
+        class="inline-flex items-center justify-center gap-2 rounded-[0.75rem] font-semibold transition-all duration-200 {{ classes }} {{ sizing }}">
+  <span>{{ text }}</span>
+  {%- if icon -%}<span class="ml-0.5" aria-hidden="true">{{ icon | safe }}</span>{%- endif -%}
+</button>
+{%- else -%}
 <a href="{{ href }}"
    class="inline-flex items-center justify-center gap-2 rounded-[0.75rem] font-semibold transition-all duration-200 {{ classes }} {{ sizing }}"
    {% if target %}target="{{ target }}"{% endif %}
@@ -17,4 +26,5 @@
   <span>{{ text }}</span>
   {%- if icon -%}<span class="ml-0.5" aria-hidden="true">{{ icon | safe }}</span>{%- endif -%}
 </a>
+{%- endif -%}
 {%- endmacro -%}

templates/marketing/base.html

@@ -51,7 +51,7 @@
       </nav>
 
       <div class="flex items-center gap-3">
-        <a href="/login" class="hidden sm:inline-block text-sm font-medium text-white/80 hover:text-white">Connexion</a>
+        <a href="/login" class="text-sm font-medium text-white/80 hover:text-white">Connexion</a>
         {% from 'macros/button.html' import button %}
         {{ button('Démarrer →', href='/signup', variant='primary', size='sm') }}
       </div>