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

manifest/firefox.json

@@ -4,6 +4,7 @@
     "version": "0.1.0",
     "description": "Minimal Ethereum wallet for Firefox",
     "permissions": ["storage", "activeTab", "alarms", "<all_urls>"],
+    "incognito": "not_allowed",
     "content_security_policy": "default-src 'self'; script-src 'self' 'wasm-unsafe-eval'; object-src 'self'; style-src 'self'; img-src 'self' data:; connect-src 'self' https: http:; frame-src 'none'; form-action 'none'; base-uri 'none'",
     "icons": {
         "16": "icons/icon16.png",