feat(cases): create a new case in a custom folder

POST /api/cases takes an optional path; Add Case > Create New gets a 'Create in a
custom folder' option with Browse. The folder is created (or an empty one filled),
scaffolded like a normal case and registered as a linked case. System, home,
credential and Codeman folders are refused; a folder with files is Link Existing's
job; a failure after the first write undoes what this call created. Admin only in
multi-user mode, like Link Existing.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

config/test-suites.ts

@@ -35,6 +35,7 @@ export const BROWSER_TEST_GLOBS = [
   'test/shift-enter-keypress.browser.test.ts',
   'test/key-tester.browser.test.ts',
   'test/webhook-settings.browser.test.ts',
+  'test/case-custom-path.browser.test.ts',
   'test/split-pane-orchestration.browser.test.ts',
   'test/split-pane-auto-collapse.browser.test.ts',
   'test/mobile-ime-preview.browser.test.ts',