fix(menus): observe sidePanel.open promise in command handler & guard against missing chrome.sidePanel

Addresses two follow-up review findings on PR #963:

1. devin-ai-integration[bot]: src/background/commands.mjs:15 had the
   same unhandled-rejection pattern as menus.mjs. The keyboard command
   path now mirrors the menus.mjs treatment: call the action
   synchronously to preserve user-gesture context, then attach a
   .catch() to the returned thenable so a rejected sidePanel.open()
   does not bubble up as an unhandled rejection.

2. coderabbitai[bot]: src/content-script/menu-tools/index.mjs:62-69
   called chrome.sidePanel.open directly, which throws synchronously
   in browsers where chrome.sidePanel is not defined (e.g. Firefox).
   Guard the call with a typeof chrome / chrome.sidePanel check and
   return a rejected Promise so the caller's .catch() handles it
   uniformly with API rejections.

Author: octo-patch

src/background/commands.mjs

@@ -12,7 +12,16 @@ export function registerCommands() {
 
     if (command in menuConfig) {
       if (menuConfig[command].action) {
-        menuConfig[command].action(true, tab)
+        // The action may return a Promise (e.g. openSidePanel returns the
+        // chrome.sidePanel.open() Promise). Keep the call synchronous so the
+        // user-gesture context is preserved, but observe the Promise so a
+        // rejection does not become an unhandled rejection in the background.
+        const result = menuConfig[command].action(true, tab)
+        if (result && typeof result.catch === 'function') {
+          result.catch((error) => {
+            console.error(`failed to run command action "${command}"`, error)
+          })
+        }
       }
 
       if (menuConfig[command].genPrompt) {

src/content-script/menu-tools/index.mjs

@@ -62,6 +62,11 @@ export const config = {
     action: (fromBackground, tab) => {
       console.debug('action is from background', fromBackground)
       if (fromBackground) {
+        // eslint-disable-next-line no-undef
+        if (typeof chrome === 'undefined' || !chrome.sidePanel?.open) {
+          // sidePanel API is not available in this browser (e.g. Firefox)
+          return Promise.reject(new Error('chrome.sidePanel API is not available'))
+        }
         // eslint-disable-next-line no-undef
         return chrome.sidePanel.open({ windowId: tab.windowId, tabId: tab.id })
       }