Feat: breakpoints CSS en @custom-media via postcss-custom-media

- postcss.config.js: plugin postcss-custom-media
- variables.css: @custom-media --mobile / --tablet / --tablet-only
- Remplacement de tous les max-width: 700px et 912px hardcodés
  par @media (--mobile), (--tablet), (--tablet-only)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/views/Expertise.svelte

@@ -363,7 +363,7 @@
   }
 
   /* Mobile (≤ 700px) */
-  @media screen and (max-width: 700px) {
+  @media (--mobile) {
     .expertise-title {
       grid-area: 5/4 / span 1 / span 7;
       font-size: var(--font-size-title-main-mobile);
@@ -379,7 +379,7 @@
   }
 
   /* Tablet (701–912px) */
-  @media screen and (min-width: 701px) and (max-width: 912px) {
+  @media (--tablet-only) {
     .expertise-title {
       grid-area: 5/6 / span 4 / span 12;
       font-size: var(--font-size-title-main-tablet);

src/views/Home.svelte

@@ -175,7 +175,7 @@
     }
   }
 
-  @media screen and (min-width: 701px) and (max-width: 912px) {
+  @media (--tablet-only) {
     .home-subtitle {
       font-size: var(--font-size-subtitle-tablet);
       width: 70%;

src/views/Play.svelte

@@ -386,7 +386,7 @@
   }
 
   /* --- Mobile (≤ 700px) --- */
-  @media screen and (max-width: 700px) {
+  @media (--mobile) {
     .game-preview {
       display: none;
     }

src/views/Portfolio.svelte

@@ -291,7 +291,7 @@
   }
 
   /* Mobile (≤ 700px) */
-  @media screen and (max-width: 700px) {
+  @media (--mobile) {
     .portfolio-gallery {
       grid-area: 1/1 / span 20 / span 20;
       opacity: 0.3;