Fix login screen UX: eliminate input conflicts, add clickable UI, improve error handling

[Copilot]

The login screen had critical UX issues: typing 'r' or 'p' in credentials triggered unwanted actions, error messages exposed technical details, form text wasn't centered, and no username persistence existed.

Changes

Input conflict resolution

• Removed bare key handlers that intercepted form input
• Replaced with mouse-driven button UI for mode switching and password visibility

Error message sanitization

• Map connection failures → "Couldn't connect to server"
• Map 401/404 → "That user wasn't found or password is incorrect"
• Map timeouts → "Server is taking too long to respond"

Form field rendering

• Center text vertically/horizontally using get_rect(center=...)
• Normalize font to freesansbold.ttf across all fields

Username persistence

• Add "Remember username" checkbox (Shift+S toggle)
• Save to .saved_username, auto-load on init
• Optimize: clear file only on state transition, not every frame

UI controls added

• Show/Hide button adjacent to password field
• Submit button (contextual "Login"/"Register" label)
• Mode toggle button ("Switch to Login"/"Switch to Register")

# Button click detection in event loop
if event.type == pygame.MOUSEBUTTONDOWN:
    if self.toggle_password_button_rect.collidepoint(mouse_pos):
        self.show_password = not self.show_password

All keyboard shortcuts (Tab/Enter/ESC) preserved.

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