Add New Zealand APRS frequency and custom frequency input

- Add New Zealand (144.575 MHz) to APRS region dropdown
- Add Argentina, Brazil, and China regions
- Add custom frequency input option for user-specified frequencies
- Custom frequency field shows/hides dynamically when selected
- Properly disable/enable custom frequency control during operation
- CSS improvements for nav element flex behavior

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

static/css/index.css

@@ -743,6 +743,7 @@ header h1 {
     align-items: center;
     gap: 12px;
     margin-left: auto;
+    flex-shrink: 0;
 }
 
 @media (min-width: 1024px) {
@@ -757,6 +758,8 @@ header h1 {
     gap: 6px;
     font-family: 'JetBrains Mono', monospace;
     font-size: 11px;
+    flex-shrink: 0;
+    white-space: nowrap;
 }
 
 .nav-clock .utc-label {
@@ -781,6 +784,7 @@ header h1 {
     display: flex;
     align-items: center;
     gap: 4px;
+    flex-shrink: 0;
 }
 
 .nav-tool-btn {