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).

static/css/main.css

@@ -0,0 +1,149 @@
+/* ESP32-Web Dashboard — dark theme overrides for Pico CSS */
+:root {
+  --pico-font-family: system-ui, -apple-system, sans-serif;
+}
+
+/* Tab navigation */
+.tab-nav {
+  display: flex;
+  gap: 0;
+  border-bottom: 2px solid var(--pico-muted-border-color);
+  margin-bottom: 1.5rem;
+}
+
+.tab-nav a {
+  padding: 0.6rem 1.2rem;
+  text-decoration: none;
+  color: var(--pico-muted-color);
+  border-bottom: 2px solid transparent;
+  margin-bottom: -2px;
+  transition: color 0.2s, border-color 0.2s;
+}
+
+.tab-nav a:hover {
+  color: var(--pico-primary);
+}
+
+.tab-nav a.active {
+  color: var(--pico-primary);
+  border-bottom-color: var(--pico-primary);
+}
+
+/* Viz containers */
+.viz-container {
+  position: relative;
+  width: 100%;
+  min-height: 400px;
+}
+
+.viz-container svg {
+  width: 100%;
+  height: 100%;
+}
+
+/* Loading state */
+.viz-loading {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  min-height: 400px;
+  color: var(--pico-muted-color);
+}
+
+/* Treemap */
+.treemap-cell {
+  stroke: var(--pico-background-color);
+  stroke-width: 1px;
+}
+
+.treemap-label {
+  fill: #fff;
+  font-size: 11px;
+  pointer-events: none;
+}
+
+/* Force graph */
+.graph-node {
+  stroke: var(--pico-background-color);
+  stroke-width: 1.5px;
+  cursor: grab;
+}
+
+.graph-node:active {
+  cursor: grabbing;
+}
+
+.graph-link {
+  stroke: var(--pico-muted-border-color);
+  stroke-opacity: 0.5;
+}
+
+.graph-label {
+  font-size: 10px;
+  fill: var(--pico-muted-color);
+  pointer-events: none;
+}
+
+/* Pack layout */
+.cluster-circle {
+  stroke: var(--pico-muted-border-color);
+  stroke-width: 1px;
+  fill-opacity: 0.7;
+}
+
+.cluster-label {
+  font-size: 11px;
+  fill: #fff;
+  pointer-events: none;
+  text-anchor: middle;
+}
+
+/* D3 tooltip */
+.d3-tooltip {
+  position: absolute;
+  padding: 0.5rem 0.75rem;
+  background: var(--pico-card-background-color, #1a1a2e);
+  border: 1px solid var(--pico-muted-border-color);
+  border-radius: 4px;
+  font-size: 0.8rem;
+  color: var(--pico-color);
+  pointer-events: none;
+  z-index: 10;
+  max-width: 300px;
+}
+
+/* Stats row */
+.stats-row {
+  display: flex;
+  gap: 1rem;
+  flex-wrap: wrap;
+  margin-bottom: 1.5rem;
+}
+
+.stat-card {
+  flex: 1;
+  min-width: 120px;
+  text-align: center;
+  padding: 1rem;
+}
+
+.stat-card .stat-value {
+  font-size: 2rem;
+  font-weight: 700;
+  color: var(--pico-primary);
+}
+
+.stat-card .stat-label {
+  font-size: 0.85rem;
+  color: var(--pico-muted-color);
+}
+
+/* Empty state */
+.viz-empty {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  min-height: 300px;
+  color: var(--pico-muted-color);
+  font-style: italic;
+}