feat(provider-form): custom User-Agent presets dropdown in advanced settings

Polish the provider-level User-Agent override UI on the Claude and Codex forms.

- Add a shared CustomUserAgentField (label + input + preset dropdown + live
  validation) so both forms stay in sync.
- Provide curated UA presets (Claude Code / Kilo Code families that pass
  coding-plan UA whitelists per #3671); the first is Claude Code's real
  `claude-cli/x (external, cli)` format. Whitelists gate on the name prefix,
  not the version, so static values stay valid across upgrades.
- Expose presets via a dropdown to the right of the input (z-[200] so it
  renders above the dialog layers) instead of inline chips.
- Move the field into the existing advanced/reasoning collapsibles.
- userAgent.ts mirrors the backend byte rule (reject only control chars;
  non-ASCII is allowed) for a non-blocking inline hint.
- i18n for all four locales (zh/en/ja/zh-TW).

tests/components/ClaudeFormFields.test.tsx

@@ -91,6 +91,8 @@ const renderCopilotForm = (overrides: Partial<ClaudeFormFieldsProps> = {}) => {
     onApiKeyFieldChange: vi.fn(),
     isFullUrl: false,
     onFullUrlChange: vi.fn(),
+    customUserAgent: "",
+    onCustomUserAgentChange: vi.fn(),
     ...overrides,
   };