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/tarifs.html

@@ -117,7 +117,7 @@
 <section class="relative bg-brand-navy text-white py-20 overflow-hidden" aria-labelledby="tarifs-cta-title">
   <div class="absolute inset-0 pointer-events-none" aria-hidden="true">
     <div class="absolute top-1/3 left-1/3 w-[500px] h-[500px] rounded-full"
-         style="background: radial-gradient(circle, rgba(124,58,237,0.12) 0%, transparent 60%); filter: blur(50px);"></div>
+         style="background: radial-gradient(circle, rgba(37,99,235,0.12) 0%, transparent 60%); filter: blur(50px);"></div>
   </div>
   <div class="relative max-w-[820px] mx-auto px-6 text-center">
     <h2 id="tarifs-cta-title" class="text-[clamp(2rem,3vw,2.75rem)] font-black mb-6">