feat: add max_claude_code_version setting and disable auto-upgrade env var

Add maximum Claude Code version limit to complement the existing minimum
version check. Refactor the version cache from single-value to unified
bounds struct (min+max) with a single atomic.Value and singleflight group.

- Backend: new constant, struct field, cache refactor, validation (semver
  format + cross-validation max >= min), gateway enforcement, audit diff
- Frontend: settings UI input, TypeScript types, zh/en i18n
- Add CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC=1 to all Claude Code
  tutorials on /keys page (unix/cmd/powershell/vscode settings.json)

frontend/src/i18n/locales/en.ts

@@ -4119,7 +4119,11 @@ export default {
         minVersion: 'Minimum Version',
         minVersionPlaceholder: 'e.g. 2.1.63',
         minVersionHint:
-          'Reject Claude Code clients below this version (semver format). Leave empty to disable version check.'
+          'Reject Claude Code clients below this version (semver format). Leave empty to disable version check.',
+        maxVersion: 'Maximum Version',
+        maxVersionPlaceholder: 'e.g. 2.5.0',
+        maxVersionHint:
+          'Reject Claude Code clients above this version (semver format). Leave empty to allow any version.'
       },
       scheduling: {
         title: 'Gateway Scheduling Settings',