Fix: fetchNote returns default empty entry instead of undefined

[swuecho]

Summary

Fixed fetchNote in services/note.ts to always return a valid DiaryEntry instead of potentially returning undefined.

Problem

When a note didn't exist (new note or offline), fetchNote returned undefined, which could cause issues in consumers like DiaryEditor.vue.

Solution

• Changed return type from Promise<DiaryEntry | undefined> to Promise<DiaryEntry>
• Returns a default empty diary entry when note doesn't exist:

  {
    noteId,
    note: '',
    dirty: false,
  }

Benefits

• Prevents undefined errors in consumers
• Better UX for new notes - editor shows empty content instead of nothing
• Type-safe: no need to handle undefined case

Testing

• Create a new note (date that doesn't exist)
• Verify editor shows empty content
• Offline mode: verify new notes still work