feat: optimize SEO based on GSC data

- Fix broken hreflang: /en/ ghost page removed, all hreflang point to canonical /
- Add canonical URL tag
- Remove emoji from page titles
- Rewrite title/description with target keywords (LaTeX OCR, math formula recognition, handwriting math)
- Add JSON-LD WebApplication structured data schema
- Update og:image to clean URL without OSS params, add og:image dimensions
- Fix favicon reference from vite.svg to favicon.png
- Add public/sitemap.xml with hreflang annotations
- Add public/robots.txt pointing to sitemap
- Update seoHelper.ts keywords for both zh/en to match search intent
- Add CLAUDE.md project documentation

public/robots.txt

@@ -0,0 +1,4 @@
+User-agent: *
+Allow: /
+
+Sitemap: https://texpixel.com/sitemap.xml

public/sitemap.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
+        xmlns:xhtml="http://www.w3.org/1999/xhtml">
+  <url>
+    <loc>https://texpixel.com/</loc>
+    <lastmod>2026-03-11</lastmod>
+    <changefreq>weekly</changefreq>
+    <priority>1.0</priority>
+    <xhtml:link rel="alternate" hreflang="zh-CN" href="https://texpixel.com/"/>
+    <xhtml:link rel="alternate" hreflang="en" href="https://texpixel.com/"/>
+    <xhtml:link rel="alternate" hreflang="x-default" href="https://texpixel.com/"/>
+  </url>
+</urlset>