Explain empty TUI views instead of showing a bare table

An account folder is only registered in the index by an import, so a freshly
configured account.toml produced an empty accounts table with nothing to say
why or what to do about it.

Each view now explains its own emptiness and names the key that resolves it,
distinguishing configured-but-not-imported from no-folders-at-all, and
telling apart a filter that matched nothing from having no data.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -61,6 +61,10 @@ money parsers                # available statement parsers
 money config                 # which data root is in use, and why
 ```
 
+An account folder appears in the app only once its statements have been
+imported — creating an `account.toml` is not enough on its own. Run
+`money import` (or press `i` in the TUI) after adding one.
+
 ### TUI keys
 
 | Key | Action |