Fork Notesium source and restructure into Go package layout

Initial fork of github.com/alonswartz/notesium into librenotes:
- Source moved to internal/notesium/ (package notesium)
- Thin entry point at cmd/librenotes/main.go
- Module renamed to git.librete.ch/public/librenotes
- main() exposed as notesium.Run()
- LICENSE preserved (MIT), NOTICE added with attribution
- Web assets and completion.bash co-located with embedding code
  to satisfy go:embed path constraints

Closes #3, #34, #35.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

NOTICE

@@ -0,0 +1,13 @@
+librenotes
+==========
+
+Cloud-native multi-tenant notes application.
+
+This project is a fork of Notesium (https://github.com/alonswartz/notesium),
+created and maintained by Alon Swartz, licensed under the MIT License.
+
+The original Notesium code is preserved under internal/notesium/ and remains
+under the MIT license. See LICENSE for the full license text.
+
+Modifications and additions made for librenotes are also released under the
+MIT License.