Refonte UI : design system, logo, shell responsive et pages simples

- Nouveaux tokens (indigo/corail, statuts, plateformes) et mode sombre
- Logo et favicon Social Flow redessinés
- AppShell partagé : sidebar groupée (desktop) + barre de navigation basse (mobile)
- Suppression des pages mobiles dupliquées et des composants morts
- Tableau de bord, connexion, 404, calendrier, médiathèque, IA, statistiques

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017ADkrVcVFTT14hbB148HYS

client/src/components/theme-provider.tsx

@@ -0,0 +1,51 @@
+import { ThemeProvider as NextThemesProvider, useTheme } from "next-themes";
+import { Monitor, Moon, Sun } from "lucide-react";
+import { cn } from "@/lib/utils";
+import type { ReactNode } from "react";
+
+export function ThemeProvider({ children }: { children: ReactNode }) {
+  return (
+    <NextThemesProvider attribute="class" defaultTheme="system" enableSystem storageKey="theme" disableTransitionOnChange>
+      {children}
+    </NextThemesProvider>
+  );
+}
+
+const OPTIONS = [
+  { value: "light", label: "Clair", icon: Sun },
+  { value: "dark", label: "Sombre", icon: Moon },
+  { value: "system", label: "Système", icon: Monitor },
+] as const;
+
+/** Sélecteur segmenté clair / sombre / système. */
+export function ThemeToggle({ className, compact = false }: { className?: string; compact?: boolean }) {
+  const { theme, setTheme } = useTheme();
+  const current = theme ?? "system";
+
+  return (
+    <div
+      role="radiogroup"
+      aria-label="Thème de l'interface"
+      className={cn("inline-flex w-full items-center gap-1 rounded-lg bg-muted p-1", className)}
+    >
+      {OPTIONS.map(({ value, label, icon: Icon }) => (
+        <button
+          key={value}
+          type="button"
+          role="radio"
+          aria-checked={current === value}
+          title={label}
+          onClick={() => setTheme(value)}
+          className={cn(
+            "flex flex-1 items-center justify-center gap-1.5 rounded-md px-2 py-1.5 text-xs font-medium text-muted-foreground transition-colors",
+            current === value ? "bg-card text-foreground shadow-sm" : "hover:text-foreground",
+          )}
+          data-testid={`theme-${value}`}
+        >
+          <Icon className="h-3.5 w-3.5" />
+          {!compact && <span>{label}</span>}
+        </button>
+      ))}
+    </div>
+  );
+}