feat: v0.1.4 — device intelligence dashboard

Add tabbed dashboard at /dashboard/ with three D3.js visualizations:
- Vendor treemap (devices grouped by type and vendor)
- SSID social graph (force-directed, shared probed SSIDs as edges)
- Fingerprint clusters (packed circles by device behavior)

Intelligence API endpoints at /api/v1/intelligence/ with param
validation. Dashboard built on htmx + Pico CSS dark theme + D3 v7,
all vendored locally (make vendor). 13 new tests (59 total).

templates/dashboard/index.html

@@ -0,0 +1,19 @@
+{% extends "base.html" %}
+{% block title %}Device Intelligence — ESP32-Web{% endblock %}
+
+{% block content %}
+<hgroup>
+  <h2>Device Intelligence</h2>
+  <p>{{ total_devices }} devices tracked across {{ total_sensors }} sensors</p>
+</hgroup>
+
+<nav class="tab-nav">
+  <a href="#" hx-get="{{ url_for('dashboard.tab_vendor_treemap') }}" hx-target="#tab-content">Vendor Treemap</a>
+  <a href="#" hx-get="{{ url_for('dashboard.tab_ssid_graph') }}" hx-target="#tab-content">SSID Graph</a>
+  <a href="#" hx-get="{{ url_for('dashboard.tab_fingerprint_clusters') }}" hx-target="#tab-content">Fingerprint Clusters</a>
+</nav>
+
+<div id="tab-content">
+  <div class="viz-loading" aria-busy="true">Loading...</div>
+</div>
+{% endblock %}