Replace the signature preset dropdown (multiple named signatures) with
a single saved signature per user, mirroring the supplier profile
behavior: Save/Load buttons, auto-loaded on the new invoice screen.
- New signature_profiles table (one row per user), migrated from any
existing signature_presets data.
- New /api/signature GET/PUT/DELETE endpoints replacing
/api/signature-presets.
- Updated frontend to save/load the single signature instead of a
named preset dropdown.
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
