docs: use absolute URLs for OG meta images

Social media crawlers require absolute URLs to resolve images correctly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/index.html

@@ -10,7 +10,7 @@
   <meta property="og:type" content="website">
   <meta property="og:title" content="Oikos — The Self-Hosted Family Planner">
   <meta property="og:description" content="A privacy-first, self-hosted family planner. Tasks, shopping, meals, calendar sync, budget — all self-hosted, no cloud dependency.">
-  <meta property="og:image" content="og-image.png">
+  <meta property="og:image" content="https://ulsklyc.github.io/oikos/og-image.png">
   <meta property="og:url" content="https://ulsklyc.github.io/oikos/">
   <meta property="og:site_name" content="Oikos">
 
@@ -18,7 +18,7 @@
   <meta name="twitter:card" content="summary_large_image">
   <meta name="twitter:title" content="Oikos — The Self-Hosted Family Planner">
   <meta name="twitter:description" content="A privacy-first, self-hosted family planner. Tasks, shopping, meals, calendar sync, budget — all self-hosted.">
-  <meta name="twitter:image" content="twitter-image.png">
+  <meta name="twitter:image" content="https://ulsklyc.github.io/oikos/twitter-image.png">
 
   <link rel="canonical" href="https://ulsklyc.github.io/oikos/">
   <link rel="icon" type="image/svg+xml" href="logo.svg">