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).

src/lib/userAgent.ts

@@ -0,0 +1,16 @@
+/**
+ * 自定义 User-Agent 合法性校验。
+ *
+ * 与后端 `parse_custom_user_agent`（基于 `http::HeaderValue::from_str`）口径严格一致：
+ * HeaderValue 按**字节**判定合法性，规则为 `b >= 32 && b != 127 || b == '\t'`。也就是说：
+ * - 制表符（\t）、可见 ASCII（0x20–0x7E）、以及任意非 ASCII 字符（UTF-8 字节均 ≥ 0x80）都合法；
+ * - 仅控制字符非法：除 \t 外的 0x00–0x1F（含换行）与 0x7F（DEL）。
+ *
+ * 空串（trim 后为空）视为"未设置"，合法。
+ */
+export function isValidUserAgentHeader(value: string): boolean {
+  const trimmed = value.trim();
+  if (trimmed === "") return true;
+  // eslint-disable-next-line no-control-regex
+  return !/[\x00-\x08\x0a-\x1f\x7f]/.test(trimmed);
+}