feat(dns): add DHCP automatic indicator and reset-to-DHCP

[awlx]

Follow-up to #200 (DoH support) as discussed.

What changed

DHCP (automatic) indicator

• Added a "DHCP (automatic)" entry at the bottom of the DNS server dropdown
• When no DNS servers are manually configured, "DHCP (automatic)" is shown as selected9; making it clear to the user that DHCP-provided DNS is in use
• Previously, the dropdown would misleadingly show "Cloudflare" as a fallback even when no custom DNS was set

Reset behavior

• Selecting "DHCP (automatic)" and clicking Apply resets DNS to automatic (same behavior as the Reset button)
• Pressing Reset now jumps the dropdown to "DHCP (automatic)" and unchecks DoT/DoH checkboxes for visual consistency
• DoT/DoH checkboxes and latency test buttons are disabled when DHCP is selected (no server to configure)

i18n

• Added dhcp-automatic string to en locale

Testing

• Start with no DNS configured → dropdown shows "DHCP (automatic)" ✓
• Set Cloudflare DoH → dropdown shows "Cloudflare" with DoH checked ✓
• Click Reset → dropdown jumps to "DHCP (automatic)", checkboxes unchecked ✓
• Select "DHCP (automatic)" + Apply → DNS reset to DHCP ✓
• Switch connections → dropdown updates correctly ✓