seo: improve SEO and fix social media link preview image

- Fix og:image and twitter:image to use absolute URLs (was the root cause of missing social preview image)
- Add og:image:width, og:image:height, og:image:alt, og:image:type
- Add og:site_name, og:locale, twitter:site, twitter:image:alt
- Add canonical link tag and robots meta
- Add JSON-LD structured data for Organization, SoftwareApplication, WebSite
- Add public/robots.txt pointing to sitemap
- Add public/sitemap.xml for search engine crawling
- Add public/site.webmanifest for PWA/mobile home screen support
- Add theme-color meta and apple-touch-icon link
- Update lang attribute to en-PH

public/site.webmanifest

@@ -0,0 +1,17 @@
+{
+  "name": "TapTrack",
+  "short_name": "TapTrack",
+  "description": "On-premises NFC student attendance for Philippine schools.",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#ffffff",
+  "theme_color": "#2563eb",
+  "icons": [
+    {
+      "src": "/favicon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any maskable"
+    }
+  ]
+}