Clean up flash key legacy code and extract import.php inline styles

App::consumeFlash() had 18-line legacy fallback chains reading from seven old
session keys (error, admin_error, edit_error, form_error, success,
admin_success, edit_success) that were written by no code in the codebase.
All action handlers have used App::flash() -> _flash_error / _flash_success
since the App class was introduced. Removed the dead fallbacks; consumeFlash()
is now 4 lines.

admin/import.php was the last admin template with inline style= attributes.
Extracted four of them to named CSS classes in admin.css:
- admin-error-list   — error <ul> spacing (was style="margin:.5rem 0 0;padding-left:1.2rem")
- admin-file-hint    — <small> display + margin (was style="margin-top:.5rem")
- admin-import-results        — results panel margin (was style="margin-top:2rem")
- admin-import-results__title — <h2> typography (was multi-property inline style)

Closes the 'unify flash message keys' item in todo/02-php-components.md and
the import.php inline style item in todo/01-css-semantic-refactor.md.

todo/02-php-components.md

@@ -21,7 +21,7 @@
 - [ ] Extract `ThesisEditController` — merges `edit.php` + `actions/edit.php`, deduplicates jury fieldset
 - [ ] Extract remaining controllers one by one
 - [ ] Consolidate action handlers into controller methods
-- [ ] Unify flash message keys project-wide to `_flash_error` / `_flash_success`
+- [x] Unify flash message keys project-wide to `_flash_error` / `_flash_success` — all callers already use `App::flash()`; removed dead legacy-key fallback chains (`error`, `admin_error`, `edit_error`, `form_error`, `success`, `admin_success`, `edit_success`) from `consumeFlash()`
 - [ ] Move OG tag construction into controller logic
 - [ ] Extract inline CSS/JS from `system.php` into separate assets