feat(desktop): implement storage management in the settings ui

[jouwdan]

What changed:

• Added collection sizing for JSON, JSONL, artifact directories, agent session index, and agent transcripts.
• Added storage tools for support-bundle export, clearing logs/audit/artifacts, deleting old screenshots, and pruning stale non-running agent sessions.
• Added a new Settings → Storage tab with collection sizes, maintenance actions, support-bundle export.
• Updated the mock renderer bridge and CLI command catalog so preview mode and CLI discovery know about the new storage tools.

Verification:

• pnpm --filter @meith/desktop typecheck
• pnpm --filter @meith/desktop test
• pnpm --filter @meith/cli typecheck
• pnpm --filter @meith/cli test
• Scoped pnpm exec biome check ... on touched files