Download page: add Windows installer link stub + btn-windows style

Adds the v0.6.2 Windows installer link pointing to the expected
filename `itsgoin-0.6.2-windows-x64-setup.exe`. Link is live before
the artifact lands so the Windows build pipeline (separate host) has
a stable path to upload to — same pattern we use to coordinate the
Linux / Android builds with the website copy.

Adds `.btn-windows { background: #0078d4 }` to style.css for the
standard four-button download row: Android, Linux AppImage, Linux
CLI, Windows Installer.

website/download.html

@@ -62,6 +62,10 @@
                     Linux CLI / Anchor
                     <span class="sub">v0.6.2</span>
                 </a>
+                <a href="itsgoin-0.6.2-windows-x64-setup.exe" class="download-btn btn-windows">
+                    Windows Installer
+                    <span class="sub">v0.6.2</span>
+                </a>
             </div>
 
             <ul style="color: var(--text-muted); font-size: 0.85rem; line-height: 1.6; margin-top: 1rem;">

website/style.css

@@ -189,6 +189,7 @@ p strong { color: var(--text); }
 .download-btn .sub { font-size: 0.82rem; font-weight: 400; opacity: 0.75; display: block; margin-top: 0.15rem; }
 .btn-android { background: var(--accent); }
 .btn-linux { background: var(--green); }
+.btn-windows { background: #0078d4; }
 
 /* --- Steps --- */
 .steps { list-style: none; padding: 0; counter-reset: step; }