Implement gpui-based gui subcommand

[pullfrog]

What changed

• Added a new gui subcommand wired into clap and command dispatch.
• Implemented a gpui-based desktop UI for existing accounts that includes:
  • account list with active selection
  • read-only 2FA code display with clipboard copy button
  • fetch and respond controls for mobile confirmations
  • fetch and respond controls for login approvals
• Kept setup/import flows out of the GUI as requested.

Implementation notes

• gui is implemented as a manifest-level command and loads accounts from the existing manifest.
• It honors --username by preselecting that account and errors if the account does not exist.
• Confirmation and approval actions include token refresh retry handling.
• Window startup failure now reports a regular command error instead of panicking.

Verification

• cargo fmt --all
• cargo check --bin steamguard
• cargo check --bin steamguard --no-default-features

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