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

Settings has a new "Allow weak passwords" checkbox, off in a new profile. 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: one Argon2id derivation at libsodium's interactive setting. README's Key Decisions now states those parameters and why one light derivation is enough: the password's length is what protects the stored secret.

The screens say "password", as README's wording rules require, so the setting is "Allow weak passwords".

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 }],