Accounts Phase 1c: sign-in UI, magic-link landing, auth store

- Shared reactive auth store (auth.user) + postJSON helper (sends the cookie).
- SignIn modal: email -> "check your inbox" (calm, no password); Google slots in
  here in Phase 2.
- /auth/verify route exchanges the magic-link token for a session, then home.
- Header shows "Sign in" or an account avatar; the You sheet gains "Signed in as
  …" + Sign out (or a Sign in row). Anonymous browsing is unchanged.

frontend/src/lib/components/Header.svelte

@@ -1,5 +1,6 @@
 <script>
-  let { onBoundaries, onHistory, filtersOn = false } = $props();
+  let { onBoundaries, onHistory, onaccount, user = null, filtersOn = false } = $props();
+  let initial = $derived((user?.display_name || user?.email || '?').trim()[0].toUpperCase());
 </script>
 
 <header class="appbar">
@@ -20,6 +21,13 @@
           stroke="currentColor" stroke-width="1.8" stroke-linecap="round" /></svg>
         <span>History</span>
       </button>
+      {#if user}
+        <button class="acct" onclick={onaccount} title={user.email} aria-label="Your account">
+          <span class="avatar">{initial}</span>
+        </button>
+      {:else}
+        <button class="signin" onclick={onaccount}>Sign in</button>
+      {/if}
     </nav>
   </div>
 </header>
@@ -49,6 +57,15 @@
   .utils button svg { width: 17px; height: 17px; }
   .utils button:hover { background: var(--accent-soft); color: var(--accent-deep); }
   .utils button.on { color: var(--accent-deep); }
+  .utils .signin { border-color: var(--line); color: var(--accent-deep); }
+  .utils .signin:hover { background: var(--accent-soft); }
+  .acct { padding: 4px; }
+  .avatar {
+    display: inline-flex; align-items: center; justify-content: center;
+    width: 30px; height: 30px; border-radius: 999px;
+    background: var(--accent); color: #fff; font-weight: 600; font-size: 0.8rem;
+    font-family: var(--label);
+  }
 
   /* On phones the utilities live in the bottom tab bar ("You") instead. */
   @media (max-width: 720px) {

frontend/src/lib/components/SignIn.svelte

@@ -0,0 +1,149 @@
+<script>
+  import { startEmail } from '$lib/auth.svelte.js';
+
+  let { onclose } = $props();
+  let email = $state('');
+  let status = $state('idle'); // idle | sending | sent | error
+  let error = $state('');
+
+  async function submit(e) {
+    e.preventDefault();
+    if (!email || status === 'sending') return;
+    status = 'sending';
+    error = '';
+    try {
+      await startEmail(email);
+      status = 'sent';
+    } catch (err) {
+      status = 'error';
+      error = err?.message || 'Something went wrong — try again in a moment.';
+    }
+  }
+</script>
+
+<div class="overlay" onclick={onclose} role="presentation">
+  <div class="sheet rise" role="dialog" aria-modal="true" aria-label="Sign in" onclick={(e) => e.stopPropagation()}>
+    <button class="x" onclick={onclose} aria-label="Close">×</button>
+
+    {#if status === 'sent'}
+      <h2>Check your inbox</h2>
+      <p class="sub">
+        We've sent a one-time sign-in link to <strong>{email}</strong>. It works once and
+        expires in 15 minutes.
+      </p>
+      <button class="primary" onclick={onclose}>Done</button>
+    {:else}
+      <h2>Sign in to Upbeat Bytes</h2>
+      <p class="sub">
+        Save articles and keep your history across devices. We'll email you a one-time
+        link — no password to remember.
+      </p>
+
+      <form onsubmit={submit}>
+        <input
+          type="email"
+          bind:value={email}
+          placeholder="you@example.com"
+          autocomplete="email"
+          required
+        />
+        <button class="primary" type="submit" disabled={status === 'sending'}>
+          {status === 'sending' ? 'Sending…' : 'Email me a link'}
+        </button>
+      </form>
+
+      {#if status === 'error'}<p class="err">{error}</p>{/if}
+      <!-- Phase 2: "Continue with Google" button slots in here -->
+      <p class="fine">No account needed to browse — signing in just saves your stuff.</p>
+    {/if}
+  </div>
+</div>
+
+<style>
+  .overlay {
+    position: fixed;
+    inset: 0;
+    background: rgba(10, 22, 38, 0.32);
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    padding: 20px;
+    z-index: 40;
+  }
+  .sheet {
+    background: var(--surface);
+    border: 1px solid var(--line);
+    border-radius: var(--radius);
+    box-shadow: var(--shadow);
+    width: 100%;
+    max-width: 400px;
+    padding: 28px 26px 22px;
+    position: relative;
+  }
+  .x {
+    position: absolute;
+    top: 12px;
+    right: 14px;
+    background: none;
+    border: none;
+    font-size: 1.5rem;
+    line-height: 1;
+    color: var(--muted);
+    cursor: pointer;
+  }
+  h2 {
+    font-size: 1.5rem;
+    margin: 0 0 8px;
+  }
+  .sub {
+    margin: 0 0 18px;
+    color: var(--muted);
+    font-size: 0.95rem;
+  }
+  form {
+    display: flex;
+    flex-direction: column;
+    gap: 10px;
+  }
+  input {
+    font: inherit;
+    padding: 11px 14px;
+    border: 1px solid var(--line);
+    border-radius: 12px;
+    background: var(--bg);
+    color: var(--ink);
+  }
+  input:focus {
+    outline: none;
+    border-color: var(--accent);
+  }
+  .primary {
+    font: inherit;
+    font-weight: 600;
+    background: var(--accent);
+    color: #fff;
+    border: none;
+    border-radius: 999px;
+    padding: 11px 16px;
+    cursor: pointer;
+    transition: background 0.14s ease;
+  }
+  .primary:hover {
+    background: var(--accent-deep);
+  }
+  .primary:disabled {
+    opacity: 0.6;
+    cursor: default;
+  }
+  .err {
+    margin: 10px 0 0;
+    color: #9a3b3b;
+    font-size: 0.86rem;
+  }
+  .fine {
+    margin: 16px 0 0;
+    color: var(--muted);
+    font-size: 0.8rem;
+    text-align: center;
+  }
+</style>