[TallblokeUK]

Fixes from a test pass over 4.1.0-beta.1.

Changes

• Safe mode: the admin screens' REST requests carry snippets-safe-mode when the page was opened in safe mode, so a snippet that fails on every request can be switched off from the Snippets list. The notice says how safe mode was turned on and links out of it.
• Snippet REST API: a create request ignores an id in its body instead of overwriting that snippet; an unknown scope is refused; deleting a locked snippet returns 409 with a reason instead of a 500.
• Priority: kept within the stored range (0–32767), and the priority inputs set the same bounds. A negative value becomes 0 rather than its absolute value.
• Validator: errors are reported against the snippet line they are on, and an unfinished declaration no longer raises a PHP warning.
• Bulk delete: "Delete Permanently" asks for confirmation and reports how many snippets were deleted.
• Import: parsing an export warns about each snippet that had a condition, as conditions are not imported.
• Settings: a checkbox setting with reworded copy shows it once, as its label.
• Export filenames: keep non-ASCII letters.
• Editor lint: a parser failure on half-typed code is no longer logged as a console error.
• Narrow screens: the toolbar tabs scroll sideways below 782px, the toolbar clears the admin bar at 600px and below, and the settings editor preview fits its cell.

Verification

• New and extended PHPUnit tests for the REST API, priority, validator line numbers, setting descriptions and the import warning. Each fails without its change.
• Full PHPUnit suite passes locally; lint:php, ESLint and Stylelint are clean.
• Checked in a browser against a local site: safe-mode recovery from the list, priority, bulk delete, settings copy, export filename, and page width at 375, 600, 782 and 1440px.