feat: add OG meta tags to home page, tag browse, and profile bio

Complete Open Graph coverage across all public pages:

- Home page: og:title (site name), og:description, og:type=website,
  og:url, og:site_name
- Tag browse: og:title with tag name, og:description with count,
  og:url, og:site_name
- Profile: add og:description using bio (truncated to 200 chars)
  with fallback to generic text

Previously only fave detail and profile (without description) had
OG tags.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/templates/pages/home.html

@@ -1,3 +1,13 @@
+{{define "head"}}
+<meta property="og:title" content="{{.SiteName}}">
+<meta property="og:description" content="Lagre og del dine favoritter">
+<meta property="og:type" content="website">
+{{if .ExternalURL}}
+    <meta property="og:url" content="{{.ExternalURL}}">
+{{end}}
+<meta property="og:site_name" content="{{.SiteName}}">
+{{end}}
+
 {{define "content"}}
 {{if .User}}
     <hgroup>