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/tracker.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/alpine.min.js" defer></script>
   <script src="/assets/tabler.min.js" defer></script>
 
@@ -156,6 +157,17 @@
               </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="18" height="18" 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 active-page" href="tracker.html">
                 <span class="nav-link-icon d-md-none d-lg-inline-block">
@@ -603,7 +615,22 @@
         _charts: { atmIv: null, rr25: null, fly25: null },
 
         async init() {
-          // expirations are built from snapshot data after loadHistory()
+          // restore last loaded history (survives navigating away & back)
+          const vs = ViewState.load('tracker');
+          if (vs) {
+            this.symbol = vs.symbol ?? this.symbol;
+            this.expirations = vs.expirations ?? [];
+            this.expiry = vs.expiry ?? '';
+            this.snapshots = vs.snapshots ?? [];
+            if (this.snapshots.length) this.$nextTick(() => this.renderCharts());
+          }
+        },
+
+        _persist() {
+          ViewState.save('tracker', {
+            symbol: this.symbol, expirations: this.expirations,
+            expiry: this.expiry, snapshots: this.snapshots,
+          });
         },
 
         async fetchExpirations() {
@@ -615,6 +642,7 @@
             const env = await res.json();
             const json = env.data ?? env;
             this.expirations = Array.isArray(json) ? json : (json.expirations || []);
+            this._persist();
           } catch (e) {
             this.expirations = [];
           }
@@ -640,6 +668,7 @@
               const expSet = new Set(this.snapshots.map(s => s.expiry).filter(Boolean));
               this.expirations = [...expSet].sort();
             }
+            this._persist();
             this.$nextTick(() => this.renderCharts());
           } catch (e) {
             this.error = e.message;