fix(notes): add self participant to new notes

[ComputelessComputer]

Create plain sessions through one helper so new notes include the current user participant while self-only blanks still clean up.

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Note

Medium Risk
Changes note/session creation and emptiness detection logic, which can affect session cleanup behavior and participant mappings across the app. Risk is moderate due to new transactional writes and default-user fallbacks, but scope is localized to session/participant creation.

Overview
New notes are now created via the shared createSession helper from useNewNote, useNewNoteAndListen, and useNewNoteAndUpload, removing the duplicated inline session-row creation and moving the note_created analytics emission into the helper.

createSession now assigns the current user (store.getValue("user_id") with a DEFAULT_USER_ID fallback) to the session and ensures the current user is added as a manual participant (creating a blank humans row if needed) in a single transaction; event-derived participant creation is also updated to use the current user.

isSessionEmpty is adjusted so a note with only the default self participant is still considered empty, while additional non-auto participants count as content. Added sessions.test.ts to cover self-participant creation, analytics firing, and the updated emptiness semantics.

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