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/assets/viewstate-store.js

@@ -0,0 +1,28 @@
+/**
+ * Per-page view-state persistence — survives navigation within the app.
+ * Exposed as window.ViewState. No modules, no deps.
+ *
+ * Each page stores its last-loaded inputs + data under
+ * localStorage["optionsPricer:view:<page>"] so navigating away and back
+ * restores the page exactly as it was.
+ */
+(function () {
+  "use strict";
+  const PREFIX = "optionsPricer:view:";
+  window.ViewState = {
+    PREFIX,
+    load(page) {
+      try {
+        const raw = localStorage.getItem(PREFIX + page);
+        return raw ? JSON.parse(raw) : null;
+      } catch { return null; }
+    },
+    save(page, data) {
+      try { localStorage.setItem(PREFIX + page, JSON.stringify(data)); }
+      catch (e) { console.warn("[ViewState] save failed:", e); }
+    },
+    clear(page) {
+      try { localStorage.removeItem(PREFIX + page); } catch {}
+    },
+  };
+})();