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

Firefox gives an extension page in a private window locks of its own, so a popup there could save between the background's read and its write and undo a network switch the background saved. `manifest/firefox.json` now sets `"incognito": "not_allowed"`, so such a page cannot read or write extension storage, and `tests/manifest.test.js` pins the key. README and the user guide say AutistMask does not work in Firefox private windows. Chrome needs no change: in its default incognito mode an extension page shares the background's lock.

A Firefox user on "Never remember history" can no longer run AutistMask at all.

Model: opus-5-5

src/popup/views/stateRecovery.js

@@ -143,8 +143,6 @@ async function resetProfile() {
     try {
         // Under the lock every write of the stored record takes, so the erase
         // cannot land between another write's read and its write and be undone.
-        // A page in a Firefox private window is the exception: Firefox gives
-        // it locks of its own (README, "Stored state and its version").
         await withLock("autistmask", () => storageRemove("autistmask"));
     } catch (e) {
         log.errorf("state recovery: reset failed:", e);