ci: migrate project automation to Just (#442)

* ci: migrate project automation to Just

Centralize build, test, release, dependency, hook, and website commands in Just recipes. Pin workflow tooling, use the repository toolchain, and run the complete validation set in CI.

* ci: codex being codex

* docs: update CLAUDE.md for just task runner migration

Reflect the switch to just recipes for build/test/clippy/fmt, note
dprint replacing raw rustfmt, and add a cross-platform code requirement.

* fix: resolve clippy warnings breaking CI on ubuntu and windows

Use clone() instead of implicit to_string() on already-owned String
values, gate the windows-only unused make_file_at import behind
cfg(posix), and fix unused mut / manual assign-op in the windows-only
localhost test.

* fix: fmt

docs/en-US/configuration/themes.md

@@ -47,20 +47,20 @@ are two quick fixes:
 1. Re-import the official theme. After each release the official themes are
    patched, so download the updated theme from the repository and re-import it:
 
-    ```sh
-    termscp theme <theme.toml>
-    ```
+   ```sh
+   termscp theme <theme.toml>
+   ```
 
 2. Edit your theme by hand. If you use a custom theme, edit the file and add the
    missing key. The theme is located at `$CONFIG_DIR/theme.toml`, where
    `$CONFIG_DIR` is:
 
-    - FreeBSD/Linux: `$HOME/.config/termscp`
-    - macOS: `$HOME/.config/termscp`
-    - Windows: `%USERPROFILE%\.termscp`
+   - FreeBSD/Linux: `$HOME/.config/termscp`
+   - macOS: `$HOME/.config/termscp`
+   - Windows: `%USERPROFILE%\.termscp`
 
-    Missing keys are reported in the CHANGELOG under `BREAKING CHANGES` for the
-    version you have just installed.
+   Missing keys are reported in the CHANGELOG under `BREAKING CHANGES` for the
+   version you have just installed.
 
 ## Styles