Update site fonts and add a French flag to the navigation bar

Update CSS to use Bahnschrift for titles and a sans-serif font for body text, and add a French flag SVG to the navbar.

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artifacts/voix-du-peuple/src/App.tsx

@@ -28,6 +28,18 @@ function Navbar() {
           <Link href="/about" className="transition-colors hover:text-foreground/80 text-foreground/60" data-testid="nav-about-link">À propos</Link>
           <Link href="/transparence" className="transition-colors hover:text-foreground/80 text-foreground/60" data-testid="nav-transparence-link">Fonctionnement</Link>
         </nav>
+        <div className="ml-auto flex items-center">
+          <span
+            title="République française"
+            aria-label="Drapeau français"
+            className="inline-flex overflow-hidden rounded-sm opacity-70"
+            style={{ width: 22, height: 15, boxShadow: "0 0 0 1px rgba(0,0,0,0.12)" }}
+          >
+            <span style={{ flex: 1, background: "#002395" }} />
+            <span style={{ flex: 1, background: "#EDEDED" }} />
+            <span style={{ flex: 1, background: "#ED2939" }} />
+          </span>
+        </div>
       </div>
     </header>
   );