feat(upgrade): add --offline flag and automatic offline fallback

Add --offline flag to 'sentry cli upgrade' that uses only cached version
info (from background checks) and cached patches (from background
prefetch) — no network calls at all.

When a normal upgrade encounters a network error during version
discovery, automatically falls back to the offline path if cached
data is available, logging a warning about the fallback.

The infrastructure for offline upgrades already existed:
- Background version checks store latest version in SQLite
- Background patch prefetch caches patch chains to disk
- Delta upgrade already checks cache before network

This change wires those pieces together with a new entry point.

Changes:
- Add --offline flag to upgrade command
- Add resolveOfflineTarget() for cached version lookup
- Add resolveTargetWithFallback() for auto network→cache fallback
- Thread offline param through executeUpgrade/downloadBinaryToTemp
  to prevent full-binary download fallback when offline
- Show '(offline, from cache)' in human output for offline upgrades
- Validate --offline is only used with curl-installed binaries

Author: BYK