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

src/popup/views/addWallet.js

@@ -132,7 +132,8 @@ function validatePassword() {
         showError("add-wallet-password-error", "Please choose a password.");
         return null;
     }
-    if (pw.length < 12) {
+    // "Allow weak passwords" in Settings lifts the 12-character minimum.
+    if (!state.allowWeakPasswords && pw.length < 12) {
         showError(
             "add-wallet-password-error",
             "Password must be at least 12 characters.",