Add strategy P/L analyzer + view-state persistence

- New strategy.html: thinkorswim-style P/L diagram (expiration + T+N curves
  via Black-Scholes, days-to-expiry slider, net debit/credit, max profit/loss
  with unbounded detection, breakevens, net Greeks, auto-detected strategy name)
- chain.html: per-row Buy/Sell buttons add legs to a localStorage basket;
  basket badge in toolbar; auto-scroll to ATM row on load
- Persist per-page view state (symbol, expiry, loaded data, charts) across
  navigation via viewstate-store.js for chain/surface/tracker/dashboard
- New assets: blackscholes.js (frontend BS port), strategy-store.js, viewstate-store.js
- Strategy P/L nav link added to all sidebars

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

frontend/surface.html

@@ -197,6 +197,17 @@
               </a>
             </li>
 
+            <li class="nav-item">
+              <a class="nav-link" href="strategy.html">
+                <span class="nav-link-icon d-md-none d-lg-inline-block">
+                  <svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+                    <path d="M4 19l4 -6l4 2l4 -8l4 5"></path><path d="M4 4v16h16"></path>
+                  </svg>
+                </span>
+                <span class="nav-link-title">Strategy P/L</span>
+              </a>
+            </li>
+
             <li class="nav-item">
               <a class="nav-link" href="tracker.html">
                 <span class="nav-link-icon d-md-none d-lg-inline-block">
@@ -557,6 +568,7 @@
     </div>
   </div>
 
+  <script src="/assets/viewstate-store.js"></script>
   <script>
     const CHART_BG       = '#1e2030';
     const CHART_GRID     = '#2d3045';
@@ -604,7 +616,24 @@
         termChartInstance: null,
 
         async init() {
-          // no auto-load — user must click Lookup first
+          // restore last loaded surface (survives navigating away & back)
+          const vs = ViewState.load('surface');
+          if (vs) {
+            this.symbol = vs.symbol ?? this.symbol;
+            this.expirations = vs.expirations ?? [];
+            this.expiry = vs.expiry ?? '';
+            if (vs.analytics) {
+              this.analytics = vs.analytics;
+              this._processAnalytics(vs.analytics);
+            }
+          }
+        },
+
+        _persist(includeAnalytics) {
+          ViewState.save('surface', {
+            symbol: this.symbol, expirations: this.expirations, expiry: this.expiry,
+            analytics: includeAnalytics ? this.analytics : null,
+          });
         },
 
         async fetchExpirations() {
@@ -622,6 +651,7 @@
             const data = env.data ?? env;
             this.expirations = data.expirations || (Array.isArray(data) ? data : []);
             if (this.expirations.length > 0) this.expiry = this.expirations[0];
+            this._persist(false);
           } catch (err) {
             this.errorMsg = 'Failed to look up symbol: ' + err.message;
           } finally {
@@ -644,6 +674,7 @@
             const data = env.data ?? env;
             this.analytics = data;
             this._processAnalytics(data);
+            this._persist(true);
           } catch (err) {
             this.errorMsg = 'Failed to load surface: ' + err.message;
           } finally {