Add favicon, richer title, and mobile viewport meta

- Inline SVG favicon (data URI): dark code-pane tile, monospace D +
  teal cursor bar — the masthead identity at 16px
- Title: "DSA Pattern Lab · FAANG Prep"; description + theme-color meta
- viewport meta was missing on the Pages deployment (the artifact
  wrapper had been providing it) — phones now render responsively

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: DeepCodeWork

index.html

@@ -1,4 +1,9 @@
-<title>DSA Pattern Lab</title>
+<title>DSA Pattern Lab · FAANG Prep</title>
+<meta charset="utf-8">
+<meta name="viewport" content="width=device-width, initial-scale=1">
+<meta name="description" content="Interactive FAANG interview prep: 25 DSA patterns with derivations, step-through trace players, flashcards, a spaced-repetition ladder, and a journal.">
+<meta name="theme-color" content="#0E1613">
+<link rel="icon" href="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%2064%2064'%3E%3Crect%20width='64'%20height='64'%20rx='12'%20fill='%230E1613'/%3E%3Crect%20x='5.5'%20y='5.5'%20width='53'%20height='53'%20rx='9'%20fill='none'%20stroke='%233FBFAB'%20stroke-opacity='0.35'%20stroke-width='2'/%3E%3Ctext%20x='12'%20y='47'%20font-family='Menlo,monospace'%20font-size='40'%20font-weight='bold'%20fill='%23E2E9E4'%3ED%3C/text%3E%3Crect%20x='40'%20y='15'%20width='9'%20height='36'%20fill='%233FBFAB'/%3E%3C/svg%3E">
 <style>
 /* ============ tokens ============ */
 :root {