Add missing translation keys for MTU and MSS Fix validation errors

[Copilot]

The validation logic in Settings.vue references error.tun_mtu_min and error.mssfix_min translation keys that were not defined, causing missing error messages when validation fails.

Changes:

• Added tun_mtu_min and mssfix_min error keys to all translation files (en, it, es)
• Error messages follow existing validation pattern for minimum value constraints

// Settings.vue validation now displays proper error messages
if (parseInt(this.tun_mtu) < 576) {
  this.error.tun_mtu = this.$t("error.tun_mtu_min");  // Now resolves correctly
}

if (parseInt(this.mssfix) < 0) {
  this.error.mssfix = this.$t("error.mssfix_min");  // Now resolves correctly
}

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[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• registry.npmmirror.com
  • Triggering command: /usr/local/bin/node node /usr/local/bin/yarn install (dns block)
  • Triggering command: /usr/local/bin/node node /usr/local/bin/npm install (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)