feat: introduce version info & update command

[irisyann]

Summary

• Adds cg update command to upgrade the CLI in-place, auto-detecting the install method (Homebrew, go install, or the install script) and prompting for confirmation before running
• Adds a passive update-available nudge on cg startup: checks GitHub Releases and prints a one-liner hint when a newer version exists
• Surfaces the current version in the welcome box header (◆ CoinGecko CLI v1.2.3)

Changes

internal/updater/checker.go (new)

Core update-check logic extracted into its own package:

• Check(currentVersion) — passive checker used at startup. Reads a 24-hour cache at ~/.config/coingecko-cli/update_check.json before hitting the network; skips entirely when CG_NO_UPDATE_CHECK=1 is set, or when the build version is dev/empty (local builds). Times out at 2 s so it never blocks the prompt.
• FetchLatest() — explicit fetch used by cg update. 10 s timeout, updates the cache on success.
• Both functions strip the leading v from the GitHub tag so version strings are compared uniformly (e.g. "1.4.0").

cmd/update.go (new)

cg update command:

• Detects install method from the resolved binary path (/Cellar/ or /opt/homebrew/ → Homebrew; $GOBIN prefix → go install; fallback → install.sh script). Can be overridden with --method homebrew|go|script.
• Validates the fetched version string against a semver regex before acting on it.
• Prompts for confirmation via huh.NewConfirm (Charm ecosystem, consistent with cg auth). Exits cleanly on Ctrl-C / "No".
• Delegates the actual upgrade to the appropriate package manager; streams stdout/stderr so progress is visible.

internal/display/banner.go

• PrintWelcomeBox now accepts the build version string and renders it in the header row.
• PrintUpdateReminder(current, latest) — new helper that writes the update nudge to stderr, respecting NO_COLOR.
• Fixed a latent padding bug: printColoredRow was calling len() on a string that may contain multi-byte runes (e.g. ◆); replaced with utf8.RuneCountInString so box borders align correctly on all terminals.

cmd/root.go

• Passes version into PrintWelcomeBox.
• Calls updater.Check after printing the welcome box and surfaces the reminder if an update is available.

Test plan

• cg (no args) — welcome box shows current version in header; no update notice when already on latest
• cg on an older build (spoof version string) — update notice appears below the welcome box
• CG_NO_UPDATE_CHECK=1 cg — no network call, no notice
• cg update when already on latest — prints "Already up to date"
• cg update when a newer version exists — shows current → latest, confirms, runs the correct install command for the detected method
• cg update --method homebrew|go|script — overrides detection, runs expected command
• cg update --method invalid — returns a clear error
• Cancel the confirmation prompt — exits cleanly with no error
• Terminal with multi-byte characters in version string — welcome box borders stay aligned