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/dashboard.html

@@ -0,0 +1,37 @@
+{{template "layout" .}}
+
+{{define "title"}}bbq — my events{{end}}
+
+{{define "content"}}
+<div class="event-header">
+  <div class="event-tag">Dashboard</div>
+  <h1 class="event-title">My events</h1>
+  <p style="font-family:'DM Mono',monospace;font-size:0.8rem;color:#555;margin-top:8px;">
+    Welcome back, {{.User.Name}}.
+  </p>
+</div>
+
+<div style="margin-bottom:24px;">
+  <a href="/" class="btn-claim" style="text-decoration:none;display:inline-block;">+ Create event</a>
+</div>
+
+{{if .Events}}
+<div class="slots-grid">
+  {{range .Events}}
+  <div class="slot-card">
+    <div class="slot-info">
+      <div class="slot-name">{{.Title}}</div>
+      <div style="font-family:'DM Mono',monospace;font-size:0.72rem;color:#555;">
+        {{.Date}} &middot; {{.Time}} &middot; {{.Location}}
+      </div>
+    </div>
+    <div class="slot-right">
+      <a href="/e/{{.Slug}}/admin/{{.AdminToken}}" class="btn-claim" style="text-decoration:none;">Manage</a>
+    </div>
+  </div>
+  {{end}}
+</div>
+{{else}}
+<p class="nobody" style="text-align:center;padding:40px 0;">No events yet. Create one above!</p>
+{{end}}
+{{end}}