feat: add website link to app footer, Join Alpha opens in new tab

Fixes #2 - Add website link to app footer
- Add VITE_WEBSITE_URL build arg for configurable website URL
- Add Git Repository and License links to app footer
- Update Join Free Alpha links to open in new tab (target=_blank)

www/index.html

@@ -19,7 +19,7 @@
         <a href="#features">Features</a>
         <a href="docs/">Docs</a>
         <a href="${GIT_URL}" target="_blank">Git Repository</a>
-        <a href="${APP_URL}" class="btn btn-primary">Join Free Alpha</a>
+        <a href="${APP_URL}" target="_blank" class="btn btn-primary">Join Free Alpha</a>
       </div>
     </div>
   </nav>
@@ -29,7 +29,7 @@
       <h1>Your AI-Powered<br><span class="gradient-text">Daily Journal</span></h1>
       <p class="hero-subtitle">Capture events throughout the day and let AI transform them into beautiful, thoughtful diary pages. Self-hosted. Private. Yours.</p>
       <div class="hero-actions">
-        <a href="${APP_URL}" class="btn btn-primary btn-large">Join Free Alpha</a>
+        <a href="${APP_URL}" target="_blank" class="btn btn-primary btn-large">Join Free Alpha</a>
         <a href="docs/" class="btn btn-secondary btn-large">Read Docs</a>
       </div>
       <div class="hero-meta">
@@ -115,12 +115,12 @@
         <h2>Ready to start journaling?</h2>
         <p>Try our free alpha hosted for you, or self-host in minutes with Docker.</p>
         <div class="try-actions">
-          <a href="${APP_URL}" class="btn btn-primary btn-large">Join Free Alpha</a>
+          <a href="${APP_URL}" target="_blank" class="btn btn-primary btn-large">Join Free Alpha</a>
           <span class="try-or">or</span>
           <a href="docs/getting-started/installation.html" class="btn btn-secondary btn-large">Self-Host</a>
         </div>
         <div class="try-code">
-          <code>git clone https://github.com/lotherk/deardiary.git && cd deardiary && docker compose up -d</code>
+          <code>git clone ${GIT_URL}.git && cd deardiary && docker compose up -d</code>
         </div>
         <p class="try-hint">Self-hosted default login: admin@localhost / changeme123</p>
       </div>
@@ -135,7 +135,7 @@
           <a href="#features">Features</a>
           <a href="docs/">Documentation</a>
           <a href="#how">How it Works</a>
-          <a href="${APP_URL}">Free Alpha</a>
+          <a href="${APP_URL}" target="_blank">Free Alpha</a>
         </div>
         <div class="footer-col">
           <h4>Resources</h4>