feat(brand): logo officiel DictIA + palette blue/cyan/fuchsia (matche le logo)

Logos officiels installés :
- static/images/dictia-logo.png (28 KB optimisé 256×256)
- static/images/dictia-logo-128.png (10 KB retina)
- static/images/dictia-logo-fullres.png (originaux conservés OG/social)
- static/images/dictia-logo.svg + dictia-logo-nom.svg (cleaned C2PA metadata)
- Header marketing/base.html : <img> 40×40 + wordmark "DictIA" + tagline "Transcription"
- Footer marketing/_footer.html : <img> 36×36 + wordmark
- Favicon mis à jour vers logo PNG

Note : SVG sources sont des PNG base64 wrappés (pas de vrais paths) — PNG utilisé
en production (8× plus léger), SVG conservé pour fallback.

Palette canonique alignée sur le logo :
- brand-b1 : #7c3aed (mauve) → #2563eb (blue-600 vibrant — primary)
- brand-b2 : #a855f7 (mauve clair) → #06b6d4 (cyan-500 — aqua mid)
- brand-b3 : #06b6d4 (aqua) → #c026d3 (fuchsia-600 — magenta accent)
- Gradient signature : linear-gradient(118deg, #2563eb, #06b6d4 52%, #c026d3)
- Box shadow CTA : rgba(37,99,235,0.28/0.42)
- 72 remplacements hex/rgba dans 5 templates marketing/legal + email service

Tests : 81 passed / 3 failed (3 échecs pré-existants /blog + trust-bar phrasing,
non liés à ce changement). 0 régression.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

templates/marketing/base.html

@@ -31,7 +31,8 @@
   <link rel="stylesheet" href="/static/css/marketing.css">
 
   <!-- Favicon -->
-  <link rel="icon" type="image/svg+xml" href="/static/images/favicon.svg">
+  <link rel="icon" type="image/png" href="/static/images/dictia-logo.png">
+  <link rel="alternate icon" type="image/svg+xml" href="/static/images/favicon.svg">
 
   {% block schema %}{% endblock %}
   {% block head_extra %}{% endblock %}
@@ -40,9 +41,17 @@
   <!-- Glassmorphism header (FlexiHub style: 62px, navy/.97 + backdrop-blur-xl + 0.045 border) -->
   <header class="fixed top-0 inset-x-0 z-50 h-[62px] bg-brand-navy/[0.97] backdrop-blur-xl border-b border-white/[0.045]">
     <div class="max-w-[1200px] mx-auto h-full px-6 flex items-center justify-between">
-      <a href="/" class="flex flex-col leading-none" aria-label="DictIA — Transcription, accueil">
-        <span class="font-black text-xl tracking-tight grad-text">DictIA</span>
-        <span class="text-[10px] uppercase tracking-[0.2em] text-white font-medium mt-0.5">Transcription</span>
+      <a href="/" class="flex items-center gap-3 leading-none" aria-label="DictIA — Transcription, accueil">
+        <img src="{{ url_for('static', filename='images/dictia-logo.png') }}"
+             alt=""
+             width="40"
+             height="40"
+             class="w-10 h-10 flex-shrink-0"
+             aria-hidden="true">
+        <span class="flex flex-col">
+          <span class="font-black text-xl tracking-tight grad-text">DictIA</span>
+          <span class="text-[10px] uppercase tracking-[0.2em] text-white font-medium mt-0.5">Transcription</span>
+        </span>
       </a>
 
       <nav class="hidden md:flex gap-8 text-sm font-medium text-white/80" aria-label="Navigation principale">