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/legal/_layout.html

@@ -27,7 +27,7 @@
     height: 4px;
     margin-top: 0.5rem;
     border-radius: 4px;
-    background: linear-gradient(118deg, #7c3aed, #a855f7 52%, #06b6d4);
+    background: linear-gradient(118deg, #2563eb, #06b6d4 52%, #06b6d4);
   }
   .legal-content h3 {
     font-size: 1.25rem; /* 20px */
@@ -60,16 +60,16 @@
   .legal-content ol { list-style-type: decimal; list-style-position: outside; }
   .legal-content li { margin-bottom: 0.35rem; }
   .legal-content a {
-    background: linear-gradient(118deg, #7c3aed, #a855f7 52%, #06b6d4);
+    background: linear-gradient(118deg, #2563eb, #06b6d4 52%, #06b6d4);
     -webkit-background-clip: text;
     background-clip: text;
     color: transparent;
     font-weight: 600;
     text-decoration: underline;
-    text-decoration-color: #7c3aed;
+    text-decoration-color: #2563eb;
   }
   .legal-content a:focus-visible {
-    outline: 2px solid #7c3aed;
+    outline: 2px solid #2563eb;
     outline-offset: 2px;
     border-radius: 2px;
   }
@@ -95,7 +95,7 @@
     background-color: #fafbfd;
   }
   .legal-content blockquote {
-    border-left: 4px solid #7c3aed;
+    border-left: 4px solid #2563eb;
     background-color: rgba(247, 249, 252, 0.6);
     padding: 0.75rem 1rem;
     margin: 1.25rem 0;
@@ -156,12 +156,12 @@
     transition: color 150ms ease, border-color 150ms ease, background-color 150ms ease;
   }
   .legal-toc a:hover {
-    background-color: rgba(124, 58, 237, 0.05);
+    background-color: rgba(37,99,235, 0.05);
   }
   .legal-toc a.is-active {
-    border-left-color: #7c3aed;
-    color: #7c3aed !important;
-    background-color: rgba(124, 58, 237, 0.06);
+    border-left-color: #2563eb;
+    color: #2563eb !important;
+    background-color: rgba(37,99,235, 0.06);
   }
   .legal-breadcrumb {
     position: sticky;