swamp update appears to stall with no progress feedback

Problem

When running swamp update, the command appears to hang with no visual feedback while it downloads and installs the new binary. The user sees nothing until the final success message appears, which can take several seconds depending on network speed. This makes it look like the command is broken or stalled.

$ swamp update
<... long pause with no output ...>
20:27:25.056   info    update               swamp updated successfully!
20:27:25.057   info    update               "20260302.135703.0-sha.7f1c73cc" → "20260302.191852.0-sha.061f496c"
20:27:25.057   info    update               SHA-256 integrity check passed

Expected behavior

Show progress feedback during the update, similar to how swamp auth login uses a Spinner with status messages like "Opening browser..." → "Securing session...".

For swamp update, the spinner could show steps like:

1. "Checking for updates..."
2. "Downloading vX.Y.Z..." (after the version check resolves)
3. "Verifying integrity..."
4. "Installing..."

Then stop the spinner before rendering the final success output.

Implementation notes

• The Spinner class (src/presentation/spinner.ts) already exists and supports start(), update(), and stop() methods
• HttpUpdateChecker.downloadAndInstall() in src/infrastructure/update/http_update_checker.ts performs the download silently with no callback
• The spinner should only be shown in log mode (not JSON mode), consistent with the auth login pattern
• The spinner writes to stderr, so it won't interfere with JSON output