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

src/popup/index.html

@@ -980,6 +980,23 @@
                     </button>
                 </div>
 
+                <div class="bg-well p-3 mx-1 mb-3">
+                    <h3 class="font-bold mb-1">Passwords</h3>
+                    <label
+                        class="text-xs flex items-center gap-1 cursor-pointer mb-1"
+                    >
+                        <input
+                            type="checkbox"
+                            id="settings-allow-weak-passwords"
+                        />
+                        Allow weak passwords
+                    </label>
+                    <p class="text-xs text-muted">
+                        Lets you choose a password shorter than 12 characters
+                        when you add a wallet.
+                    </p>
+                </div>
+
                 <div class="bg-well p-3 mx-1 mb-3">
                     <h3 class="font-bold mb-1">Tracked Tokens</h3>
                     <p class="text-xs text-muted mb-2">