Add Rails backend: accounts via magic link, server documents, sharing

The app becomes a Rails 8 application; the Vue frontend moves to
app/frontend and is served through vite_rails.

- Without an account documents stay in the browser (IndexedDB). Sign-in
  is by magic link or the 6-digit code from the same mail; afterwards
  local documents can be uploaded into the account.
- Server documents (/d/<id>) with versioned saves and a single-editor
  lock per browser tab; others see them read-only and follow by polling.
- Sharing via link (works without an account) and via e-mail invitation,
  roles viewer/editor; the owner manages links and people.
- Store gains a read-only mode that blocks all content changes.
- Solid Queue, Cache and Cable share the one PostgreSQL database.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/frontend/components/TheLogo.vue

@@ -0,0 +1,46 @@
+<script setup lang="ts">
+/**
+ * Numolus brand mark: three top-aligned, slightly overlapping pop-art squares
+ * with a keyline. Fills are fixed brand colours; the keyline uses a token so it
+ * stays visible in dark mode. Pass `word` to also render the "numolus" wordmark.
+ */
+defineProps<{ word?: boolean }>();
+</script>
+
+<template>
+  <svg
+    class="logo"
+    :class="{ 'has-word': word }"
+    :viewBox="word ? '64 62 270 206' : '64 64 270 142'"
+    xmlns="http://www.w3.org/2000/svg"
+    role="img"
+    aria-label="Numolus"
+  >
+    <g stroke="var(--logo-keyline, #111)" stroke-width="6">
+        <rect x="70"  y="100" width="40" height="90" style="fill:rgb(255,212,0);"/>
+        <rect x="115" y="100" width="65" height="90" style="fill:rgb(0,87,255);"/>
+        <rect x="185" y="100" width="130" height="90" style="fill:rgb(232,56,79);"/>
+    </g>
+    <text
+      v-if="word"
+      x="72"
+      y="255"
+      font-size="46"
+      font-weight="600"
+      fill="currentColor"
+    >
+      numolus
+    </text>
+  </svg>
+</template>
+
+<style scoped>
+.logo {
+  display: block;
+  height: 100%;
+  width: auto;
+}
+.logo text {
+  font-family: var(--font-ui);
+}
+</style>