Switch to phone auth via Twilio SMS and add feature flag system

Replace email-based auth (Resend) with phone-based auth (Twilio SMS).
Add BBQ_FEATURES env var for toggling features at deploy time — when
auth is disabled, no login routes are registered and the app works
as before.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/name.html

@@ -0,0 +1,24 @@
+{{template "layout" .}}
+
+{{define "title"}}bbq — your name{{end}}
+
+{{define "content"}}
+<div class="event-header">
+  <div class="event-tag">Account</div>
+  <h1 class="event-title">What's your name?</h1>
+  <p style="font-family:'DM Mono',monospace;font-size:0.8rem;color:#555;margin-top:8px;">
+    This will be used to autofill your name when you RSVP or claim a slot.
+  </p>
+</div>
+
+<div class="claim-form-wrapper">
+  <form method="POST" action="/account/name">
+    <div class="form-row">
+      <label>Name</label>
+      <input type="text" name="name" placeholder="Your name" required autofocus
+             value="{{if .User}}{{.User.Name}}{{end}}">
+    </div>
+    <button class="btn-submit" type="submit">Save &#8599;</button>
+  </form>
+</div>
+{{end}}