Add user registration/login, persistent likes, category hiding, and contact email

- User auth: register (nickname + password + email), login, logout with PHP sessions
- Persistent likes: logged-in users' likes saved to users.json, anonymous via session
- "Tykkäämäni" filter button next to search — filter liked posts, combinable with search
- Hide empty/sparse categories from filter buttons until posts exist
- Replace broken contact form with simple mailto link (info@tykkaa.fi)

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

index.html

@@ -17,6 +17,7 @@
         <h1 class="pixel-logo"><span class="logo-heart">♥</span>tykkää<span class="logo-fi">.fi</span></h1>
       </div>
       <p class="tagline" data-i18n="tagline"></p>
+      <div class="user-area" id="userArea"></div>
       <nav>
         <a href="#posts" data-i18n="nav_posts"></a>
         <a href="#about" data-i18n="nav_about"></a>
@@ -29,7 +30,10 @@
   <!-- SEARCH & FILTER -->
   <section class="controls" id="posts">
     <div class="container">
-      <input type="text" id="search" data-i18n-ph="search_ph" oninput="filterPosts()" />
+      <div class="search-row">
+        <input type="text" id="search" data-i18n-ph="search_ph" oninput="filterPosts()" />
+        <button class="likes-filter-btn" id="likesFilterBtn" onclick="toggleLikesFilter()">❤️ Tykkäämäni</button>
+      </div>
       <div class="filters" id="categoryFilters"></div>
       <div class="filters sub-filters" id="subCategoryFilters"></div>
       <button class="add-post-btn" onclick="openSubmitModal()">✏️ Lisää julkaisu</button>
@@ -60,12 +64,7 @@
     <div class="container">
       <h2 data-i18n="contact_title"></h2>
       <p data-i18n="contact_desc"></p>
-      <form class="contact-form" onsubmit="handleSubmit(event)">
-        <input type="text" data-i18n-ph="name_ph" required />
-        <input type="email" data-i18n-ph="email_ph" required />
-        <textarea data-i18n-ph="msg_ph" rows="4" required></textarea>
-        <button type="submit" class="btn" data-i18n="send_btn"></button>
-      </form>
+      <a href="mailto:info@tykkaa.fi" class="contact-email">info@tykkaa.fi</a>
     </div>
   </section>
 
@@ -190,6 +189,14 @@
     <img id="lightboxImg" src="" alt="" onclick="event.stopPropagation()" />
   </div>
 
+  <!-- AUTH MODAL -->
+  <div class="modal-overlay" id="authOverlay" onclick="closeAuthModal()">
+    <div class="modal" onclick="event.stopPropagation()" style="max-width:420px">
+      <button class="modal-close" onclick="closeAuthModal()">✕</button>
+      <div id="authContent"></div>
+    </div>
+  </div>
+
   <script src="script.js"></script>
 </body>
 </html>