Fix NaN handling in mssfix and tun_mtu validation

[Copilot]

Addresses feedback from #131: validation for mssfix and tun_mtu was accepting empty/non-numeric input because parseInt(value) < min evaluates to false when parseInt() returns NaN.

Validation logic

• Add isNaN() check before numeric comparison
• Store parsed value to avoid redundant parseInt() calls
• Use explicit radix parameter parseInt(value, 10) throughout

Translation strings

• Add missing error.tun_mtu_min and error.mssfix_min keys to EN/IT/ES

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

// After
const mssfixValue = parseInt(this.mssfix, 10);
if (isNaN(mssfixValue) || mssfixValue < 0) {
  this.error.mssfix = this.$t("error.mssfix_min");
  // ...
}

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