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)

backend/internal/handler/dto/settings.go

@@ -79,6 +79,7 @@ type SystemSettings struct {
 	OpsMetricsIntervalSeconds    int    `json:"ops_metrics_interval_seconds"`
 
 	MinClaudeCodeVersion string `json:"min_claude_code_version"`
+	MaxClaudeCodeVersion string `json:"max_claude_code_version"`
 
 	// 分组隔离
 	AllowUngroupedKeyScheduling bool `json:"allow_ungrouped_key_scheduling"`