Add AlphaPulse brand icon across the web app

Use a shared SVG combining an A mark with a market pulse line for the favicon, login page, and navigation. Preserve the existing theme and page layout.

web/index.html

@@ -3,6 +3,7 @@
   <head>
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
     <title>AlphaPulse</title>
   </head>
   <body>