fix(rdp): prompt for session credentials on mobile

[ZacharyZcR]

Related to Termix-SSH/Support#1372.

Mobile requests an RDP token immediately, even when the host requires credentials at connection time. Unlike the web client, it has no prompt and cannot send promptedUsername, promptedPassword, or promptedDomain, so those hosts reach the backend with empty login credentials.

Add an RDP sign-in dialog before requesting a token. Read legacy 2.8 authentication fields and 2.9 protocolAuth summaries; hosts using saved credentials or a personal credential override continue connecting automatically. Pass temporary credentials through the existing compatibility route. Canceling does not request a token, reconnecting prompts again, and the password input is cleared on submission or cancellation. No host credentials are saved.

Validation:

• Four new regression tests cover legacy and modern authentication modes, personal overrides, domain precedence, and the actual API request payload. Added to PR CI.
• Existing API compatibility and terminal authentication tests: 15 passed.
• Remote desktop resolution and viewport checks, TypeScript, formatting, and Expo config generation passed.
• ESLint completed with no errors and 80 warnings.

No iOS device or live RDP server was available for end-to-end validation. This fixes a confirmed missing mobile flow; the reporter's authentication mode and guacd logs are still needed to establish whether it explains Support#1372, so this PR does not close that issue.