feat: folder upload support with directory structure

[awaaate]

Description

Support uploading folders (directories) while preserving the directory structure as metadata on FileEntry.

Requirements

• Folder upload via button (webkitdirectory) and drag-and-drop (webkitGetAsEntry)
• Preserve relative paths as metadata (path field on FileEntry)
• Allow mixing loose files and folders in same upload
• Files stored flat on disk — path is metadata only
• Display folder hierarchy in SessionList UI

Files to modify

• apps/api/src/storage/types.ts — add path?: string to FileEntry
• apps/api/openapi.yaml — update schema
• apps/api/src/storage/sessionStore.ts — thread path through store functions
• apps/api/src/api/server.ts — extract paths from FormData
• packages/sdk/src/index.ts — send paths in upload
• apps/ui/src/components/UploadZone.tsx — folder input + drag-drop traversal
• apps/ui/src/components/SessionList.tsx — tree display