Persist draft attachments across navigation

[mcintyre94]

Summary

• Adds draftAttachmentPaths: [String]? to SpriteChat SwiftData model
• saveDraft now persists attached file paths alongside draft text; clears to nil when empty
• loadSession restores attachedFiles from persisted paths (reconstructing AttachedFile via lastPathComponent)
• ChatView triggers saveDraft on attachedFiles.count changes, in addition to text changes

SwiftData handles the new optional column automatically — no migration needed.

Test plan

• Attach a file in chat, navigate away, return — attachment chip should reappear
• Send the message after returning — file path is correctly prefixed as before
• Navigate away without attaching anything — no regressions to empty state
• New unit tests: saveDraft_persistsAttachmentPaths, saveDraft_clearsAttachmentPathsWhenEmpty, loadSession_restoresDraftAttachments, loadSession_doesNotOverwriteExistingAttachments

🤖 Generated with Claude Code

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.