Show currently-computed path on saveinput if valid

[tconbeer]

re-use space used by the validation error label

Show fully-resolved path, but try to show it relative to ~ if that works.

e.g.:

• user types ./foo, show ~/projects/data_project/foo if cwd is ~/projects/data_project
• user types ./foo, show /tmp/abc123/foo if cwd is /tmp/abc123

see https://github.com/tconbeer/harlequin/discussions/511

[tconbeer]

see this event handler:

https://github.com/tconbeer/textual-textarea/blob/a4c3291c77c48192f2f2c1aa2d8fbdc988d15bf3/src/textual_textarea/text_editor.py#L1132-L1139