Phase 2: Web dashboard with SSE real-time push

- dashboard.go: SSE hub + HTTP server + price history ring buffer
- static.go: //go:embed for static files
- web/static/index.html: Full dashboard HTML (6 panels)
- web/static/app.js: SSE client, Chart.js price chart, live table updates
- web/static/style.css: GitHub-style dark theme
- main.go: Start dashboard on :8888 + wire price recording + scan results

Dashboard features:
  - Real-time price table (6 coins × 4 exchanges)
  - Arbitrage opportunities table
  - Open positions view
  - Historical trades table (from SQLite)
  - Chart.js price chart with coin/exchange selector
  - Stats summary (total/converged/diverged/flat)
  - 🚫 Zero external Go dependencies (Chart.js loaded from CDN)

web/static/style.css

@@ -0,0 +1,163 @@
+/* ============================================================
+   Exchange Monitor Dashboard — Dark Theme
+   ============================================================ */
+
+:root {
+  --bg: #0d1117;
+  --card: #161b22;
+  --border: #30363d;
+  --text: #c9d1d9;
+  --text-dim: #8b949e;
+  --accent: #58a6ff;
+  --green: #3fb950;
+  --red: #f85149;
+  --yellow: #d29922;
+  --blue: #58a6ff;
+}
+
+* { margin: 0; padding: 0; box-sizing: border-box; }
+
+body {
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif;
+  background: var(--bg);
+  color: var(--text);
+  font-size: 14px;
+  line-height: 1.5;
+  min-height: 100vh;
+}
+
+#app { max-width: 1440px; margin: 0 auto; padding: 16px; }
+
+/* Header */
+header {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  padding: 12px 16px;
+  background: var(--card);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  margin-bottom: 16px;
+}
+
+header h1 { font-size: 18px; font-weight: 600; }
+.header-meta { display: flex; align-items: center; gap: 8px; font-size: 13px; color: var(--text-dim); }
+.sep { color: var(--border); }
+.status-offline { color: var(--red); }
+.status-online { color: var(--green); }
+
+/* Grid layout */
+.grid {
+  display: grid;
+  grid-template-columns: 1fr 1fr;
+  gap: 12px;
+}
+.card-wide { grid-column: 1 / -1; }
+
+/* Cards */
+.card {
+  background: var(--card);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  padding: 12px;
+}
+
+.card h2 {
+  font-size: 14px;
+  font-weight: 600;
+  color: var(--text-dim);
+  margin-bottom: 10px;
+  padding-bottom: 8px;
+  border-bottom: 1px solid var(--border);
+}
+
+/* Stats row */
+.stats-row {
+  display: flex;
+  gap: 16px;
+  flex-wrap: wrap;
+}
+.stat {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  min-width: 60px;
+}
+.stat label { font-size: 11px; color: var(--text-dim); margin-bottom: 2px; }
+.stat span { font-size: 20px; font-weight: 700; }
+.pct-green { color: var(--green); }
+.pct-red { color: var(--red); }
+.pct-gray { color: var(--text-dim); }
+.pct-yellow { color: var(--yellow); }
+.pct-blue { color: var(--blue); }
+
+/* Tables */
+.table-wrap {
+  overflow-x: auto;
+  max-height: 320px;
+  overflow-y: auto;
+}
+table {
+  width: 100%;
+  border-collapse: collapse;
+  font-size: 13px;
+}
+th {
+  text-align: left;
+  padding: 6px 8px;
+  color: var(--text-dim);
+  font-weight: 500;
+  font-size: 11px;
+  text-transform: uppercase;
+  letter-spacing: 0.5px;
+  position: sticky;
+  top: 0;
+  background: var(--card);
+  border-bottom: 1px solid var(--border);
+}
+td {
+  padding: 5px 8px;
+  border-bottom: 1px solid rgba(48, 54, 61, 0.5);
+  white-space: nowrap;
+}
+tr:hover td { background: rgba(88, 166, 255, 0.05); }
+.loading { text-align: center; color: var(--text-dim); padding: 20px !important; }
+
+.text-green { color: var(--green); }
+.text-red { color: var(--red); }
+.text-yellow { color: var(--yellow); }
+.text-dim { color: var(--text-dim); }
+.text-right { text-align: right; }
+
+/* Chart controls */
+.chart-controls {
+  display: flex;
+  gap: 8px;
+  margin-bottom: 10px;
+}
+.chart-controls select {
+  background: var(--bg);
+  color: var(--text);
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  padding: 4px 8px;
+  font-size: 13px;
+  cursor: pointer;
+}
+.chart-container {
+  position: relative;
+  height: 300px;
+}
+
+/* Scrollbar */
+::-webkit-scrollbar { width: 6px; height: 6px; }
+::-webkit-scrollbar-track { background: transparent; }
+::-webkit-scrollbar-thumb { background: var(--border); border-radius: 3px; }
+::-webkit-scrollbar-thumb:hover { background: #484f58; }
+
+/* Responsive */
+@media (max-width: 768px) {
+  .grid { grid-template-columns: 1fr; }
+  header { flex-direction: column; gap: 8px; }
+  .stats-row { justify-content: center; }
+}