WebUI: per-signal vscale toolbar, active-signal highlighting, zoom fix

- Per-signal, per-plot vertical scale state (sigVScale keyed by plotId:signalKey)
  so the same signal in two plots has fully independent vscale config
- Active signal redrawn on top of all series with 2× line width for clear
  visual identification; badge click toggles selection and opens/closes the
  embedded vscale toolbar (click same badge again to deselect)
- Vscale configurator moved from floating popup to a slim toolbar strip
  anchored inside the plot card, with an × close button
- Trigger dropdown shows one entry per array signal with [0…N-1] label;
  opening it shows an index-picker dialog to choose the element
- Zoom resampling: when server returns no data for a zoomed range, fall
  back to the local circular buffer instead of returning empty arrays

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Client/WebUI/static/index.html

@@ -158,6 +158,39 @@
         <span class="ctx-range-val" id="ctx-marker-size-val">4px</span>
       </div>
     </div>
+    <!-- ── Array index picker (trigger signal) ──────────────────────── -->
+    <div id="array-idx-picker" style="display:none">
+      <div class="ctx-menu-header">Element index: <span id="aip-sig" class="ctx-menu-key"></span></div>
+      <div class="ctx-row">
+        <label>Index</label>
+        <input type="number" id="aip-idx" class="ctx-num" min="0" step="1" value="0">
+        <span id="aip-range" style="font-size:10px;color:var(--overlay0)"></span>
+      </div>
+      <div class="ctx-row" style="justify-content:flex-end;gap:6px">
+        <button class="ctx-btn" id="aip-cancel">Cancel</button>
+        <button class="ctx-btn active" id="aip-ok">OK</button>
+      </div>
+    </div>
+    <!-- ── VScale toolbar (moved into plot card when active) ─────────── -->
+    <div id="vscale-menu" style="display:none">
+      <div class="vstb-header">
+        <span class="vstb-label">V-Scale: <span id="vscale-menu-key" class="ctx-menu-key"></span></span>
+        <div class="ctx-btns" id="vscale-mode-btns">
+          <button class="ctx-btn active" data-mode="auto">Auto</button>
+          <button class="ctx-btn" data-mode="range">Range</button>
+          <button class="ctx-btn" data-mode="manual">Manual</button>
+        </div>
+        <div id="vscale-manual-row" style="display:none;align-items:center;gap:4px">
+          <label class="vstb-lbl">V/div</label>
+          <input type="number" id="vscale-vdiv" class="ctx-num" min="1e-30" step="any" value="1">
+        </div>
+        <div id="vscale-pos-row" style="display:none;align-items:center;gap:4px">
+          <label class="vstb-lbl">Pos</label>
+          <input type="number" id="vscale-pos" class="ctx-num" step="0.1" value="0">
+        </div>
+        <button id="btn-vscale-close" class="vstb-close" title="Close">✕</button>
+      </div>
+    </div>
     <script src="/app.js"></script>
   </body>
 </html>