feat(ui): installer-style terminal UX package (internal/ui)

[aptracebloc]

Goal

Give every command the look-and-feel of the tracebloc/client one-line installer — colored step output, ✔/⚠/· feedback, dim hints, a branded banner — via a small shared package. TTY-aware so piped/CI output stays clean.

Scope

• New internal/ui package porting the installer vocabulary from tracebloc/client/scripts/lib/common.sh:
  • Step(n, total, label), Success, Warn, Info, Error, Hint, PromptHeader, Banner.
  • Palette via fatih/color matching the installer (cyan/green/yellow/red/dim/bold).
• TTY + color gating: reuse the isTTY approach from internal/push/progress.go; honor NO_COLOR (https://no-color.org) and a global --plain flag; color only when stdout is a real terminal.
• Refactor the ad-hoc fmt.Fprintf output in cluster info and dataset push pre-flight onto the new helpers (presentation only — no behavior change).

Acceptance criteria

• internal/ui with the helper set + unit tests (color-on vs plain via an injectable writer / force-color toggle).
• --plain and NO_COLOR both force plain; non-TTY auto-plains.
• cluster info + dataset push pre-flight render via ui; existing tests pass/updated.
• make ci green.

Go concepts we'll cover

• A UX interface with TTY-aware implementations (same shape as the existing Progress interface in internal/push/progress.go).
• ANSI/SGR escape basics + the NO_COLOR convention.
• Adding a dependency cleanly (fatih/color, go.mod/go.sum discipline).

Notes

• Decision locked: fatih/color (not lipgloss/bubbletea). Branch off develop; conventional commits; one PR.

Part of the UX/interactive epic.

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