fix(tui): use alternate screen buffer for login screen

[echobt]

Problem

Customers reported that on Ubuntu terminals, the TUI login screen was not rendering properly - instead of showing the interactive TUI, it would spam 'Welcome to Cortex CLI v0.0.6' messages repeatedly.

Root Cause

The login screen was using an 'inline' rendering approach that relied on cursor escape sequences (MoveUp/MoveDown) to redraw the content in place. This approach is not reliable across all terminal emulators:

• Some terminals don't properly support CSI cursor control sequences
• SSH sessions may filter or not properly handle these escape sequences
• Certain terminal multiplexer configurations (tmux, screen) can interfere
• Ubuntu's default GNOME Terminal has specific quirks with inline cursor control

Each render frame would print 12 lines of content and then attempt to move the cursor back up. When the cursor move failed, each frame added more lines instead of overwriting, causing the 'spam' behavior.

Solution

Refactored the login screen to use ratatui with the alternate screen buffer, matching the approach already used by the trust screen:

1. Uses EnterAlternateScreen for reliable full-screen rendering
2. Leverages ratatui's proper terminal abstraction and layout system
3. Uses Terminal::draw() for atomic frame rendering
4. Works consistently across all terminal emulators

Changes

• Converted from raw crossterm cursor manipulation to ratatui's Terminal abstraction
• Used proper Layout and Constraint system for positioning
• Maintained the same visual design (colors, text, options)
• Properly handles alternate screen entry/exit for clean terminal restoration

Testing

• Verified cargo check passes for cortex-cli and cortex-tui
• Verified cargo fmt passes
• The fix follows the same pattern as trust_screen.rs which is known to work correctly