[UI] Default fleet in project wizard

[olgenn]

Close https://github.com/dstackai/dstack-cloud/issues/373

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Note

Enables creating fleets directly from the UI and during project creation, with validation, summaries, and help content.

• New FleetAdd wizard (routing under projects/:projectName/fleets/add) with form fields, Yup validation, help panel info, and summary step
• Project creation flows (ProjectAdd, CreateProjectWizard) include an optional "Default fleet" step; when enabled, call FLEETS_APPLY to create the fleet
• API: adds PROJECTS.FLEETS_APPLY endpoint and useApplyFleetMutation; extends fleet and project types for apply/create payloads
• Global confirmation dialogs: Redux slice (components/ConfirmationDialog/slice), useConfirmationDialog hook, and rendering in AppLayout
• New FormToggle component for RHF-Integrated toggles; export from components index
• i18n updates (e.g., "OK", "Create", fleet labels/messages); ConfirmationDialog confirm label defaults to OK
• UX tweaks: internal navigation for "Create a fleet" alert, "Create" button in project list, improved ButtonWithConfirmation defaults

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