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

README.md

@@ -1787,18 +1787,20 @@ view would leave a wallet one click from deletion.
       without it, the lost-password route on DeleteWallet is the first they
       would hear of it. The hint line reserves its height, so switching tabs
       cannot move the password fields under the pointer.
+    - The password needs at least 12 characters. With "Allow weak passwords"
+      ticked in Settings, one character is enough.
     - "Import" button, with the error line beside it so that it adds no height
       to a screen whose button already starts near the bottom of the popup
 - **Transitions**:
-    - "Import" with a valid entry and a matching password of at least 12
-      characters → creates the wallet, clears the navigation stack, and →
-      **Home**. When no address is active yet, as on the first wallet, the new
-      wallet's first address becomes the active one. The phrase and xprv modes
-      then scan for further used addresses and report the count as a flash
-      message. When the scan finds two or more used addresses, they replace the
-      wallet's list, which then holds the first address only if it was used. If
-      the first address was active and is not in the new list, the new list's
-      first address becomes the active one and `AUTISTMASK_ACTIVE_CHANGED` is
+    - "Import" with a valid entry and a matching password that is long enough →
+      creates the wallet, clears the navigation stack, and → **Home**. When no
+      address is active yet, as on the first wallet, the new wallet's first
+      address becomes the active one. The phrase and xprv modes then scan for
+      further used addresses and report the count as a flash message. When the
+      scan finds two or more used addresses, they replace the wallet's list,
+      which then holds the first address only if it was used. If the first
+      address was active and is not in the new list, the new list's first
+      address becomes the active one and `AUTISTMASK_ACTIVE_CHANGED` is
       broadcast.
     - "Import" with a missing, short or mismatched password → full-sentence
       error on the error line, no screen change
@@ -2235,6 +2237,10 @@ view would leave a wallet one click from deletion.
     - Wallets: one row per wallet with its name (tap to rename inline), a
       `[recovery phrase]` button on HD wallets only, and an `[x]` delete button,
       plus a "+ Add wallet" button
+    - Passwords:
+        - "Allow weak passwords" checkbox, unticked until the user ticks it
+        - A sentence saying the setting lets the user choose a password shorter
+          than 12 characters on AddWallet
     - Tracked Tokens: one row per token tracked on the active network with an
       `[x]` remove button, plus a "+ Add token" button
     - Display: "Show tracked tokens with zero balance" checkbox, "UTC
@@ -3061,6 +3067,11 @@ logged.
     - The user's password is run through Argon2id (`crypto_pwhash`) to derive a
       256-bit encryption key. Argon2id is memory-hard, making GPU/ASIC brute
       force attacks expensive.
+    - Each use of the password runs one Argon2id derivation at libsodium's
+      interactive setting: `OPSLIMIT_INTERACTIVE` and `MEMLIMIT_INTERACTIVE`, 2
+      passes over 64 MiB. Every signature asks for the password, so a light
+      setting keeps signing quick. The password's length, not the derivation's
+      cost, is what protects the stored secret.
     - The derived key encrypts the secret material using XSalsa20-Poly1305
       (`crypto_secretbox`), which provides authenticated encryption (the
       ciphertext cannot be tampered with without detection).