Security hardening, bug fixes, and per-tab alert icon

- Prevent fake finding injection via per-session nonce validation between
  MAIN world interceptor and ISOLATED world content script
- Fix CSV formula injection in export by sanitizing cell values
- Serialize storage writes to prevent race conditions across tabs
- Cap findings at 5000 with oldest-first eviction
- Delete findings by unique ID instead of URL to avoid collateral removal
- Validate keyword length (50 chars) and count (50 max)
- Add MutationObserver for SPA support (dynamic DOM scanning)
- Add explicit CSP to manifest
- Add per-tab alert icon with red dot overlay when secrets are found

js/interceptor-loader.js

@@ -0,0 +1,16 @@
+(function () {
+  "use strict";
+
+  const nonce = crypto.randomUUID();
+
+  // Store nonce where both MAIN world (interceptor) and ISOLATED world (content.js) can read it.
+  // The interceptor removes data-kf-nonce after reading; data-kf-verify stays for content.js.
+  const el = document.documentElement;
+  el.setAttribute("data-kf-nonce", nonce);
+  el.setAttribute("data-kf-verify", nonce);
+
+  const script = document.createElement("script");
+  script.src = chrome.runtime.getURL("js/interceptor.js");
+  (document.head || document.documentElement).appendChild(script);
+  script.onload = () => script.remove();
+})();