restructure project following best practices

https://go.dev/doc/modules/layout
This also enables us to later import just the db interaction part
say to the gemini capsule backend go project.

internal/ui/templates.go

@@ -0,0 +1,15 @@
+package ui
+
+const defaultTemplate = `title: todo
+date: 2025-12-31
+slug: todo
+tags: cat, dog
+---
+
+Example text
+
+=> https://travisshears.com example link
+
+* Example list item 1
+* Example list item 2
+`