fix(desktop): clean stale draft stores

[Brendonovich]

Summary

• remove empty scoped desktop store files on startup and after store delete/clear
• prune stale/capped draft store files while preserving non-empty workspace stores
• add unit coverage for store cleanup behavior

Test

• bun test src/main/store-cleanup.test.ts
• bun typecheck (packages/desktop)
• bun turbo typecheck (pre-push hook)