Polish the dcd install script and CLI progress loader #90

Summary

Give the dcd install experience a visual polish pass. Two related surfaces:

1. The install script — install.sh (served via curl -fsSL https://get.devicecloud.dev/install.sh | sh)
2. The CLI progress loader — src/utils/progress.ts (the @clack/prompts spinner wrapper used across commands)

Both work correctly today; this is purely about making them look nicer / feel more premium.

install.sh

Currently all output goes through a plain info() / printf '%s\n'. Ideas to make it prettier:

• A small branded header/banner at the top (ASCII wordmark or a single tasteful line) so it's obvious the install started.
• Color + symbols (guarded on TTY + NO_COLOR): green ✓ for success, dim grey for secondary lines like from: / to:, yellow ! for the shadowing-install warning, red for errors.
• Group the "Resolving latest version…" → "Installing…" → download → "✓ Installed" steps into clear visual stages rather than a flat wall of lines.
• Nicer final "next steps" block (PATH hint / dcd --help) — visually set apart from the install log.

Constraints to respect:

• Must stay POSIX sh (script is #!/usr/bin/env sh, set -eu), no bash-isms.
• Keep the curl | sh safety property (everything wrapped in main(), invoked on the last line).
• Degrade gracefully with no colors when not a TTY / NO_COLOR is set / dumb terminal.
• Don't change the --progress-bar download to anything that breaks non-interactive/CI installs.

Progress loader (src/utils/progress.ts)

• Review spinner frames, colors, and status-line styling for a more polished look, consistent with src/utils/styling.ts / ui.ts.
• Ensure the CI fallback (one plain line per distinct status) still reads well and stays clean.

Acceptance

• Install output looks noticeably more polished in an interactive terminal.
• No regressions in CI / non-TTY / NO_COLOR environments.
• Still POSIX-sh clean and curl | sh-safe.