feat: an "Allow weak passwords" setting lets a new wallet have a short password (closes #674)

Settings gains a Passwords section with an "Allow weak passwords" checkbox. It
is off in a new profile, and only a stored true turns it on. While it is off,
AddWallet refuses a password shorter than 12 characters, as before. While it is
on, AddWallet accepts any password that is not empty. Key derivation is
unchanged.

The README describes the setting under Settings and AddWallet. Its Key
Decisions section now states the Argon2id parameters and why they are light.
The user guide lists the setting under Settings.

Model: opus-5-5

tests/persistedFieldContract.test.js

@@ -629,8 +629,15 @@ const CONTRACT = [
         // the falsy answer is also the DEFAULT_STATE default.
         falsy: [0],
     })),
-    // Each lets a request reach another service, so anything but true is off.
-    ...["useBlockscout", "checkRecipientsOnEtherscan"].map((field) => ({
+    // Anything but true is off for these three settings:
+    // - useBlockscout and checkRecipientsOnEtherscan let a request reach
+    //   another service.
+    // - allowWeakPasswords lets AddWallet accept a short password.
+    ...[
+        "useBlockscout",
+        "checkRecipientsOnEtherscan",
+        "allowWeakPasswords",
+    ].map((field) => ({
         field,
         kind: KIND.SCALAR,
         hostile: [42, "notabool", { a: 1 }],