android auth: add step-based email/register/2fa flow and startup route

android/CHANGELOG.md

@@ -765,3 +765,18 @@
   - group invites.
 - Removed direct `block by user id` controls from Settings UI as requested.
 - Removed extra bottom Settings actions (`Profile` row and `Back to chats` button) and kept categorized section layout.
+
+### Step 113 - Auth flow redesign (email -> password/register -> 2FA) + startup no-flicker
+- Added step-based auth domain/use-cases for:
+  - `GET /api/v1/auth/check-email`
+  - `POST /api/v1/auth/register`
+  - login with optional `otp_code` / `recovery_code`.
+- Updated Android login UI to multi-step flow:
+  - step 1: email input,
+  - step 2: password for existing account or register form (`name`, `username`, `password`) for new account,
+  - step 3: 2FA OTP/recovery code when backend requires it.
+- Improved login error mapping for 2FA-required responses, so app switches to OTP step instead of generic invalid-password message.
+- Removed auth screen flash on startup:
+  - introduced dedicated `startup` route with session-check loader,
+  - delayed auth/chats navigation until session check is finished.
+- Added safe fallback in `MainActivity` theme bootstrap to prevent crash if `ThemeRepository` injection is unexpectedly unavailable during startup.