feat: per-link IP whitelist, clipboard fallback, and link editing

- Add `allowed_ips` to Link model with IPv4/IPv6 and CIDR support
- Validate client IP in proxy auth middleware against link whitelist
- Extract client IP from X-Forwarded-For / X-Real-Ip headers
- Fix copy button for non-HTTPS contexts via execCommand fallback
- Allow editing existing links (name, type, auth mode, rate limit, IPs)
- Add dedicated IP whitelist modal for quick editing

via [HAPI](https://hapi.run)

Co-Authored-By: HAPI <[email protected]>

internal/config/config.go

@@ -39,6 +39,7 @@ type Link struct {
 	AuthMode    string   `json:"auth_mode"` // single, dual
 	Enabled     bool     `json:"enabled"`
 	RateLimit   int      `json:"rate_limit"` // requests per minute
+	AllowedIPs  []string `json:"allowed_ips"` // empty = allow all
 	CreatedAt   int64    `json:"created_at"`
 	AccessCount int64    `json:"access_count"`
 	LastAccess  int64    `json:"last_access"`