feat(app): sign in with a password and TOTP on the device

ADR 0017, step 3: halkyon_app's side, which Learn and Todo take.

- The account's cryptography on the device (`SodiumAccountCrypto`):
  halkyon_keys and SRP's client half, each step on an isolate of its
  own. sodium hands out results as views of native memory its
  finalizers free once the isolate that made them ends, so everything
  crossing back is copied first: a view read freed memory and crashed
  the process.
- `PasswordGateway` on the module's endpoints (steps without a session
  through a client holding none; an authenticator set up with a session
  granted but not yet the device's), and `PasswordRepository` running
  the flows: signing in (M2 checked, the keys opened before anything is
  trusted), signing up, a password for someone signed in, an account
  never finished, the authenticators, recovery codes, changing the
  password, a new recovery phrase, and getting back in.
- The unlocked keys in the system's keyring per instance and person
  (`SecureAccountKeyStore`), gone once another person signs in or
  nobody, and with the instance's removal.
- The screens: the start screen's Sign in and Create an account (and
  when GitHub sign-in ends), the sign-in sheet (codes, recovery codes,
  an account never finished, a lost authenticator), the account's setup
  (the recovery phrase shown once, three random words typed back before
  anything reaches the instance; the authenticator's QR code, drawn
  with pub.dev's qr; the recovery codes), getting back in, and the
  account section (`AccountSecuritySection`).
- `FileSaver` moves from Learn into halkyon_app, for the phrase and the
  codes in Todo too; `AppDependencies` names it and `passwords`.
- halkyon_keys rebuilds account keys from the secret keys a device kept;
  the key proof's message is in halkyon_base_client, checked against
  the register's.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_flutter/linux/flutter/generated_plugins.cmake

@@ -4,6 +4,7 @@
 
 list(APPEND FLUTTER_PLUGIN_LIST
   desktop_webview_window
+  file_selector_linux
   flutter_secure_storage_linux
   url_launcher_linux
   window_to_front