fix: handle missing share files in Session.getShare and ShareNext.get

[Brandtweary]

Summary

• Fix unhandled promise rejections caused by Storage.read throwing NotFoundError when checking share status for unshared sessions
• The TUI calls these functions during sync, resulting in ~1 error/second in the logs

Changes

• Add .catch(() => undefined) to Session.getShare() and ShareNext.get() to gracefully handle missing share files
• Consistent with similar patterns elsewhere in the codebase (e.g. project.ts:95, summary.ts:181)