Phase 1.2: session login + empty HTMX dashboard

- Session login for the single user Sam: server-side session store with an
  HMAC-signed session cookie, POST /login, POST /logout, and double-submit
  CSRF on every form post. Unauthenticated requests to / and /partials/*
  redirect to /login?next=...
- Empty dashboard shell in the Family Home Lab design system: left nav
  (category and folder placeholders), centre list, right detail pane.
- Copy tokens.css and the self-hosted Inter fonts from the family-home-lab
  design system into web/static; vendor htmx 2.0.4. No cross-project imports
  at runtime.
- Dark mode via prefers-color-scheme plus a persisted toggle; phone-first
  layout with 44px touch targets, hairline borders, 8px grid, pill CTAs.
- Config: EMAIL_AGENT_PASSWORD for the login password, falling back to
  EMAIL_AGENT_SESSION_SECRET so one configured secret is enough.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

web/templates/dashboard.html

@@ -0,0 +1,97 @@
+<!doctype html>
+<html lang="en" data-theme="light">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <title>Email Agent</title>
+  <link rel="stylesheet" href="/static/tokens.css">
+  <link rel="stylesheet" href="/static/app.css">
+  <script>
+    (function () {
+      try {
+        var t = localStorage.getItem("ea-theme");
+        if (!t) t = matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
+        document.documentElement.setAttribute("data-theme", t);
+      } catch (e) {}
+    })();
+    function eaToggleTheme() {
+      var el = document.documentElement;
+      var next = el.getAttribute("data-theme") === "dark" ? "light" : "dark";
+      el.setAttribute("data-theme", next);
+      try { localStorage.setItem("ea-theme", next); } catch (e) {}
+    }
+  </script>
+</head>
+<body>
+  <header class="nav-bar">
+    <span class="wordmark">Email Agent</span>
+    <div class="nav-actions">
+      <button class="btn btn-utility theme-toggle" type="button" onclick="eaToggleTheme()" aria-label="Toggle dark mode">
+        <span class="icon-light" aria-hidden="true">&#9790;</span>
+        <span class="icon-dark" aria-hidden="true">&#9728;</span>
+      </button>
+      <form method="post" action="/logout">
+        <input type="hidden" name="csrf" value="{{.CSRFToken}}">
+        <button class="btn btn-utility" type="submit">Log out</button>
+      </form>
+    </div>
+  </header>
+
+  <main class="page">
+    <h1 class="greeting">{{.Greeting}}, Sam</h1>
+
+    <div class="shell">
+      <aside class="panel" aria-label="Categories and folders">
+        <div class="nav-groups">
+          <div class="nav-group">
+            <div class="nav-group-title">Categories</div>
+            <div class="nav-group-items">
+              {{range .Categories}}
+              <a class="nav-item" href="#">
+                <span class="dot" style="background: {{.Color}}"></span>
+                <span>{{.Name}}</span>
+                <span class="count">0</span>
+              </a>
+              {{end}}
+            </div>
+          </div>
+          <div class="nav-group">
+            <div class="nav-group-title">Folders</div>
+            <div class="nav-group-items">
+              {{range .Folders}}
+              <a class="nav-item" href="#">
+                <span class="dot" style="background: {{.Color}}"></span>
+                <span>{{.Name}}</span>
+                <span class="count">0</span>
+              </a>
+              {{end}}
+            </div>
+          </div>
+        </div>
+      </aside>
+
+      <section class="panel" aria-label="Message list">
+        <div class="panel-head">
+          <h2>Inbox</h2>
+          <button class="btn btn-utility" type="button"
+                  hx-get="/partials/inbox" hx-target="#inbox-list" hx-swap="innerHTML">
+            Refresh
+          </button>
+        </div>
+        <div id="inbox-list" class="msg-list"
+             hx-get="/partials/inbox" hx-trigger="load" hx-swap="innerHTML"></div>
+      </section>
+
+      <aside class="panel detail-pane" aria-label="Message detail">
+        <div class="panel-head"><h2>Message</h2></div>
+        <div class="detail-body">
+          <p class="muted">Select a message to read it here.</p>
+          <p class="faint" style="margin-top: var(--space-xs);">Recommended action, reply draft, and history will appear in this pane.</p>
+        </div>
+      </aside>
+    </div>
+  </main>
+
+  <script src="/static/htmx.min.js" defer></script>
+</body>
+</html>