Add reasoning cache and viewer

resources/reasonButton.js

@@ -0,0 +1,34 @@
+(function() {
+  function addButton() {
+    const toolbar = document.querySelector("#header-view-toolbar") ||
+                    document.querySelector("#mail-toolbox toolbar");
+    if (!toolbar || document.getElementById('sortana-reason-button')) return;
+    const button = document.createXULElement ?
+          document.createXULElement('toolbarbutton') :
+          document.createElement('button');
+    button.id = 'sortana-reason-button';
+    button.setAttribute('label', 'Show Reasoning');
+    button.className = 'toolbarbutton-1';
+    const icon = browser.runtime.getURL('resources/img/brain.png');
+    if (button.setAttribute) {
+      button.setAttribute('image', icon);
+    } else {
+      button.style.backgroundImage = `url(${icon})`;
+      button.style.backgroundSize = 'contain';
+    }
+    button.addEventListener('command', async () => {
+      const tabs = await browser.tabs.query({ active: true, currentWindow: true });
+      const tabId = tabs[0]?.id;
+      const msgs = tabId ? await browser.messageDisplay.getDisplayedMessages(tabId) : [];
+      if (!msgs.length) return;
+      const url = browser.runtime.getURL(`reasoning.html?mid=${msgs[0].id}`);
+      browser.tabs.create({ url });
+    });
+    toolbar.appendChild(button);
+  }
+  if (document.readyState === 'complete' || document.readyState === 'interactive') {
+    addButton();
+  } else {
+    document.addEventListener('DOMContentLoaded', addButton, { once: true });
+  }
+})();