* fix(buffer): use `default_category` config for empty placeholder
Problem: The empty-buffer fallback hardcoded the category name `TODO`,
ignoring the user's `default_category` config value (default: `Todo`).
Solution: Read `config.get().default_category` at render time and use
that value for both the header line and `LineMeta` category field.
* fix(diff): match optional checkbox char in `parse_buffer` patterns
Problem: `parse_buffer` used `%[.%]` which requires exactly one
character between brackets, failing to parse empty `[]` checkboxes.
Solution: Change to `%[.?%]` so the character is optional, matching
`[]`, `[ ]`, `[x]`, and `[!]` uniformly.
7ad27f6fca