chore(branding): adopt project-manager EnCoach logo across login and sidebars

- Replace public/logo-icon.png with the approved EnCoach wordmark
  (icon + "EnCoach" + tagline "Unlock your potential with AI powered platform").
- Login page shows the full branded logo at h-36 and drops the duplicate
  text heading since the wordmark is now baked into the asset.
- Student/Teacher/Admin sidebars show a small rounded icon when collapsed
  and the full wordmark at h-14 when expanded, removing the previous
  duplicated "EnCoach" text span.
- og:image in index.html already points at /logo-icon.png so social cards
  pick up the new branding automatically.

Made-with: Cursor

src/pages/Login.tsx

@@ -70,12 +70,12 @@ export default function Login() {
   return (
     <div className="min-h-screen flex items-center justify-center bg-background p-4">
       <div className="w-full max-w-md">
-        <div className="flex items-center justify-center gap-3 mb-8">
-          <img src="/logo.png" alt="EnCoach" className="h-12 w-12 rounded-xl object-contain" />
-          <h1 className="text-2xl font-bold tracking-tight">
-            <span className="text-[hsl(42,40%,62%)]">En</span>
-            <span className="text-[hsl(240,20%,30%)]">Coach</span>
-          </h1>
+        <div className="flex items-center justify-center mb-8">
+          <img
+            src="/logo.png"
+            alt="EnCoach — Unlock your potential with AI powered platform"
+            className="h-36 w-auto object-contain"
+          />
         </div>
 
         <Card className="shadow-lg border-0 bg-card">