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>

web/templates/pages/profile.html

@@ -2,6 +2,11 @@
 {{with .Data}}{{$d := .}}{{with .ProfileUser}}
     {{if eq .ProfileVisibility "public"}}
         <meta property="og:title" content="{{.DisplayNameOrUsername}} sine favoritter">
+        {{if .Bio}}
+            <meta property="og:description" content="{{truncate 200 .Bio}}">
+        {{else}}
+            <meta property="og:description" content="{{.DisplayNameOrUsername}} sine favoritter på {{$.SiteName}}">
+        {{end}}
         <meta property="og:type" content="profile">
         {{if $.ExternalURL}}
             <meta property="og:url" content="{{$.ExternalURL}}/u/{{.Username}}">

web/templates/pages/tag_browse.html

@@ -1,3 +1,15 @@
+{{define "head"}}
+{{with .Data}}
+<meta property="og:title" content="Favoritter med merkelapp: {{.TagName}}">
+<meta property="og:description" content="{{.Total}} offentlige favoritter med merkelappen «{{.TagName}}»">
+<meta property="og:type" content="website">
+{{if $.ExternalURL}}
+    <meta property="og:url" content="{{$.ExternalURL}}/tags/{{.TagName}}">
+{{end}}
+<meta property="og:site_name" content="{{$.SiteName}}">
+{{end}}
+{{end}}
+
 {{define "content"}}
 {{with .Data}}
 <hgroup>