fix: AutistMask does not work in Firefox private windows (closes #646)

Firefox gives an extension page in a private window locks of its own. A
popup there could save between the background's read and its write of the
stored record, and undo a network switch the background had saved.

manifest/firefox.json now sets "incognito": "not_allowed", so a page in a
private window cannot read or write extension storage. tests/manifest.test.js
pins the key. The README says this in "Stored state and its version". Six
code comments drop their private-window exception to the lock.
docs/README.md tells Firefox users that AutistMask does not work in private
windows.

Model: opus-5-5

docs/README.md

@@ -208,6 +208,8 @@ password.
 3. Click "Load Temporary Add-on".
 4. Select `dist/firefox/manifest.json`.
 
+AutistMask does not work in Firefox private windows.
+
 ## Getting Started
 
 ### Creating a New Wallet