feat(upgrade): add progress spinners for version check and download phases (#515)

Show animated spinners during the two long-running phases of `sentry cli
upgrade` that previously had no visual feedback:

1. **"Checking for updates..."** — wraps the version resolution network
call (`resolveTargetWithFallback`)
2. **"Downloading {version}..."** — wraps the binary download in both
`executeStandardUpgrade` and `migrateToStandaloneForNightly`

Uses the existing `withProgress` utility from `polling.ts` for a
consistent look with all other CLI commands. Spinners are automatically
suppressed in `--json` mode.

The setup phase (`runSetupOnNewBinary`) is intentionally **not** wrapped
since it spawns a child process with `stdio: "inherit"` that writes its
own output.

### Before / After

```
# Before (several seconds of silence between messages):
ℹ Upgrading to 0.5.0...
✓ Upgraded to 0.5.0 (from 0.4.0)

# After:
⠙ Checking for updates...
⠋ Downloading 0.5.0...
✓ Upgraded to 0.5.0 (from 0.4.0)
  Method: curl · Channel: stable
```

Author: BYK