Add management API handlers for config and auth file management

- Implemented CRUD operations for authentication files.
- Added endpoints for managing API keys, quotas, proxy settings, and other configurations.
- Enhanced management access with robust validation, remote access control, and persistence support.
- Updated README with new configuration details.

Fixed OpenAI Chat Completions for codex

internal/api/handlers/management/quota.go

@@ -0,0 +1,18 @@
+package management
+
+import "github.com/gin-gonic/gin"
+
+// Quota exceeded toggles
+func (h *Handler) GetSwitchProject(c *gin.Context) {
+	c.JSON(200, gin.H{"switch-project": h.cfg.QuotaExceeded.SwitchProject})
+}
+func (h *Handler) PutSwitchProject(c *gin.Context) {
+	h.updateBoolField(c, func(v bool) { h.cfg.QuotaExceeded.SwitchProject = v })
+}
+
+func (h *Handler) GetSwitchPreviewModel(c *gin.Context) {
+	c.JSON(200, gin.H{"switch-preview-model": h.cfg.QuotaExceeded.SwitchPreviewModel})
+}
+func (h *Handler) PutSwitchPreviewModel(c *gin.Context) {
+	h.updateBoolField(c, func(v bool) { h.cfg.QuotaExceeded.SwitchPreviewModel = v })
+}