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/index.html

@@ -9,6 +9,7 @@
   <link rel="stylesheet" href="/assets/tabler.min.css">
   <link rel="stylesheet" href="/assets/tabler-vendors.min.css">
   <script src="/assets/apexcharts.min.js"></script>
+  <script src="/assets/viewstate-store.js"></script>
   <script src="/assets/tabler.min.js" defer></script>
   <script src="/assets/alpine.min.js" defer></script>
 
@@ -200,6 +201,18 @@
               </a>
             </li>
 
+            <!-- Strategy P/L -->
+            <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" aria-hidden="true">
+                    <path d="M4 19l4 -6l4 2l4 -8l4 5"/><path d="M4 4v16h16"/>
+                  </svg>
+                </span>
+                <span class="nav-link-title">Strategy P/L</span>
+              </a>
+            </li>
+
             <!-- Tracker -->
             <li class="nav-item">
               <a class="nav-link" href="tracker.html">
@@ -741,7 +754,16 @@
 
         // ---- lifecycle ----
         async init() {
+          // restore last symbol/expiry so navigating back keeps your selection
+          const vs = ViewState.load('dashboard');
+          if (vs) {
+            this.symbol = vs.symbol ?? this.symbol;
+            this.expiry = vs.expiry ?? '';
+          }
           await this.fetchExpirations();
+          if (this.expiry && !this.expirations.includes(this.expiry)) {
+            this.expiry = this.expirations[0] || '';
+          }
           if (this.expiry) await this.loadData();
         },
 
@@ -783,6 +805,7 @@
             this.snapshots = snapshotEnv.data?.snapshots ?? snapshotEnv.snapshots ?? [];
 
             this.lastUpdated = new Date().toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' });
+            ViewState.save('dashboard', { symbol: this.symbol, expiry: this.expiry });
 
             // Wait a tick for DOM, then render charts
             await this.$nextTick();