ci: run the workspace tests on every change; drop the unused flake

- .github/workflows/ci.yml runs `cargo test --workspace --locked` on
  pushes to main and on pull requests, with the Linux libraries the
  release build installs. No formatter check: the tree is not
  rustfmt-clean and stays that way.
- flake.nix, flake.lock and the direnv hook are removed. Nothing used
  them, and their pinned dependency hashes were already stale after the
  codec and kernel bumps.
- Under cfg(test) the "system" language resolves to en-US. The unit
  tests compare English text through one process-wide loader, and every
  apply_config of a default config asked for the system language, so a
  Turkish locale failed 22 tests and parallel tests raced each other
  through a language switch (the thickness-row test found its section
  under one language and looked it up under another).
- The arc grip test follows #1352: the start, end and midpoint grips
  drive the whole arc.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/i18n.rs

@@ -158,6 +158,13 @@ impl std::fmt::Display for Language {
 }
 
 fn system_languages() -> Vec<i18n_embed::unic_langid::LanguageIdentifier> {
+    // The unit tests compare English prompts and messages through one
+    // process-wide loader, and every `apply_config` of a default config asks
+    // for the system language, so the machine's locale must never reach
+    // them — not even from a test that runs in parallel.
+    #[cfg(test)]
+    return vec!["en-US".parse().expect("en-US is a valid language identifier")];
+
     #[cfg(target_arch = "wasm32")]
     let mut requested = WebLanguageRequester::requested_languages();
     #[cfg(not(target_arch = "wasm32"))]