Implement left/right voting on entries

GET /entry/{pid}/interactions/{mode} now reads the vote tally and, for
mode=left/right (auth required), toggles the user's vote: new vote, repeat
same mode removes it, different mode switches. Returns {left, right,
selected}. A UNIQUE(uid, pid) constraint on the vote table prevents
duplicate votes. The JS frontend renders Links/Rechts buttons with live
counts under each entry.

This completes the voting feature that was left commented-out and broken
in the Flask version.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/js/app.js

@@ -83,10 +83,42 @@ function renderEntry(e) {
     <div class="meta"><strong>${escapeHtml(e.username)}</strong> · ${when}</div>
     <div class="content">${linkify(e.content)}</div>
     ${media}
+    <div class="interactions">
+      <button class="vote" data-mode="left">Links</button>
+      <span class="counts">– / –</span>
+      <button class="vote" data-mode="right">Rechts</button>
+    </div>
   `;
+  setupVoting(el, e.pid);
   return el;
 }
 
+function setupVoting(el, pid) {
+  const box = el.querySelector(".interactions");
+  const counts = box.querySelector(".counts");
+  const buttons = box.querySelectorAll(".vote");
+
+  function render(state) {
+    if (state.error) {
+      counts.textContent = state.error;
+      return;
+    }
+    counts.textContent = `${state.left} / ${state.right}`;
+    buttons.forEach((b) =>
+      b.classList.toggle("selected", b.dataset.mode === state.selected)
+    );
+  }
+
+  buttons.forEach((b) =>
+    b.addEventListener("click", async () => {
+      render(await api.get(`/entry/${pid}/interactions/${b.dataset.mode}`));
+    })
+  );
+
+  // Anfangszustand laden, ohne zu werten.
+  api.get(`/entry/${pid}/interactions/none`).then(render);
+}
+
 async function loadMore() {
   if (loading || done) return;
   loading = true;