feat: 1-to-1 message render + web data-lake backend

frontend/src/routes/app/+page.svelte

@@ -0,0 +1,23 @@
+<div class="placeholder">
+  <span class="pill">Select a chat to view its archive</span>
+</div>
+
+<style lang="scss">
+  .placeholder {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    height: 100%;
+  }
+
+  .pill {
+    padding: 0.4375rem 1rem;
+    border-radius: 1rem;
+
+    font-size: 0.875rem;
+    color: var(--color-white);
+
+    background-color: var(--color-default-shadow);
+    backdrop-filter: blur(8px);
+  }
+</style>