[UX] Extend dstack login with interactive selection of url and default project

[peterschmidt85]

1. Similar to gh auth login, if the user does not pass --url to dstack login, the command prompts the user to enter it interactively. The default value in this case is https://sky.dstack.ai.

2. If the logged-in user has more than one project, dstack login prompts the user to select a default project using an interactive menu.

3. Similar to dstack project, to explicitly avoid interactive prompts, dstack login now accepts -n/--no (does not change the default project) and -y/--yes (sets the user’s first project as the default).

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Note

Improves dstack login UX with interactive prompts and explicit non-interactive flags.

• Prompts for --url when running in a TTY (default https://sky.dstack.ai); adds rich-styled UrlPrompt and graceful Ctrl-C handling
• When multiple SSO providers exist in a TTY, prompts to select provider; otherwise requires -p/--provider
• Adds -y/--yes and -n/--no to control default project selection; configures all projects first, then sets default based on flags/TTY conditions (single project auto-selected; interactive menu via select_default_project when supported)
• Tweaks output messages (e.g., "Logged in as", "Added ... at ") and centralizes isatty checks
• Wraps confirm_ask with KeyboardInterrupt handling; simplifies project.py Questionary import and cleans selection code
• Extensive tests added/updated for new flows and messaging

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Missing confirmation message after interactive project selection

Low Severity

The interactive project selection path (the else branch) successfully configures and saves the selected default project but does not print a confirmation message. All other code paths that set a default project (the --yes flag at lines 193-195 and the auto-default case at lines 224-226) print a "Set [project] project as default at [path]" message. This inconsistency means users who interactively select a project receive no feedback that their selection was applied.