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

docs/README.md

@@ -425,6 +425,8 @@ see everything unfiltered.
 Click the gear icon on the home screen to access settings:
 
 - **Wallets**: Your wallets, and "+ Add wallet".
+- **Passwords**: Tick "Allow weak passwords" to choose a password shorter than
+  12 characters when you add a wallet.
 - **Tracked Tokens**: The ERC-20 tokens tracked across all addresses, and "+ Add
   token".
 - **Display**: Toggle whether tracked tokens with zero balance are shown, switch