feat: add waveform download and storage.

sfm/waveform_viewer.html

@@ -244,6 +244,46 @@
   let eventList = [];        // populated from /device/events after connect
   let currentEventIndex = 0;
 
+  // ── DB mode: opened via ?db_id=<uuid>&api_base=<url> from History tab ────────
+  const _urlParams  = new URLSearchParams(window.location.search);
+  const _dbId       = _urlParams.get('db_id');
+  const _dbApiBase  = (_urlParams.get('api_base') || '').replace(/\/$/, '');
+
+  async function _loadFromDb() {
+    const apiBase = _dbApiBase || document.getElementById('api-base').value.replace(/\/$/, '');
+    setStatus('Loading waveform from database…', 'loading');
+    document.getElementById('unit-bar').style.display = 'none';
+    // Hide live-device controls — not relevant in DB mode
+    document.querySelector('header .conn-group').style.display = 'none';
+
+    const url = `${apiBase}/db/events/${encodeURIComponent(_dbId)}/waveform`;
+    let data;
+    try {
+      const resp = await fetch(url);
+      if (!resp.ok) {
+        const err = await resp.json().catch(() => ({ detail: resp.statusText }));
+        throw new Error(err.detail || resp.statusText);
+      }
+      data = await resp.json();
+    } catch (e) {
+      setStatus(`Error: ${e.message}`, 'error');
+      return;
+    }
+    lastData = data;
+    renderWaveform(data);
+  }
+
+  // Auto-load when opened with db_id param
+  window.addEventListener('DOMContentLoaded', () => {
+    if (_dbId) {
+      // Pre-fill api-base if provided
+      if (_dbApiBase) {
+        document.getElementById('api-base').value = _dbApiBase;
+      }
+      _loadFromDb();
+    }
+  });
+
   function setStatus(msg, cls = '') {
     const bar = document.getElementById('status-bar');
     bar.textContent = msg;
@@ -404,8 +444,11 @@
     bar.innerHTML = '';
     bar.className = 'ok';
     const ts = data.timestamp;
-    if (ts) {
-      bar.textContent = `Event #${data.index}  —  ${ts.display}    `;
+    const tsDisplay = ts
+      ? (typeof ts === 'string' ? ts : (ts.display ?? JSON.stringify(ts)))
+      : null;
+    if (tsDisplay) {
+      bar.textContent = `Event #${data.index}  —  ${tsDisplay}    `;
     } else {
       bar.textContent = `Event #${data.index}    `;
     }