feat(ui): an empty state offers the thing it is describing

[youhide]

C3b, the half of C3 that changes behaviour. #85 made there be one place to put an action; this puts one there.

Two states that could carry an action, and now do

A clean working directory said "the working directory matches the last commit" and left you to go find that commit. Its own doc comment claimed it said "what to do next" — it did not. It offers the commit now.

A file filter matching nothing said so and stopped. It offers to clear itself.

Both dispatch messages that already existed, so nothing new is plumbed through the update loop. This is a button where there was a sentence, not a feature.

An action is offered only when it exists

An unborn branch has no last commit, so that state keeps the sentence and no button. One that selected a commit which is not there would be worse than none, and a test holds that open.

What is deliberately left alone

The panes that say "Select a file to see its changes". Their next action is clicking a row in the list beside them, and a button repeating that is chrome, not help.

The strings, and what they cost

They go through the catalogue rather than inline, which is what ADR-0008 asks of new user-facing text, and both locales get them.

That needed the catalogue to reach two widgets it did not: detail::view and staging::view each take it now, threaded from the screen that already had it.

Worth flagging rather than hiding: staging::view was already eleven arguments behind an #[allow(clippy::too_many_arguments)], and this makes it thirteen. Bundling them into a struct is the right answer and is not this change.

Sabotage — four, four distinct failures

Sabotage	What broke
the clean state goes back to describing only	a_clean_working_directory_offers_the_commit_it_matches and offering_the_last_commit_actually_selects_it
the button selects the working directory instead	offering_the_last_commit_actually_selects_it
an unborn branch gets a button anyway	an_unborn_branch_has_no_commit_to_offer
the filter's action label is emptied	a_filter_that_matches_nothing_offers_to_clear_itself

The second one matters most: without it, a button that renders but goes nowhere would pass.

A coverage gap these tests closed

Nothing had ever asserted the clean-working-directory screen. Rewriting its wording broke no test — the whole suite stayed green through a copy change to a state every user with a committed repository sees. That is why two of the four are about it.

Screenshots

staging.png retaken, showing the new empty state on a fresh clone. The other three are unchanged.

Gate

cargo fmt --all -- --check, cargo clippy --workspace --all-targets --all-features --locked -- -D warnings, cargo test --workspace --all-features --locked — 904 tests, all green.