feat: improve shipping zone UX

- Remove asterisks from UK/US country names (#153)
- Remove unclear "Flat rate" option (#153)
- Alphabetize country list, move China to correct position (#153)
- Rename "Unit" to "Currency" (#154)
- Rename cost label to "Default shipping cost" (#154)
- Add currency suffix to cost field (#154)
- Add hint about per-product shipping costs (#154)
- Add validation for whole number sats (#155)
- Disable submit button when sats validation fails (#155)
- Show error message for invalid sats values (#155)

Closes #153, #154, #155

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

static/components/shipping-zones.js

@@ -19,7 +19,6 @@ window.app.component('shipping-zones', {
       currencies: [],
       shippingZoneOptions: [
         'Free (digital)',
-        'Flat rate',
         'Worldwide',
         'Europe',
         'Australia',
@@ -27,6 +26,7 @@ window.app.component('shipping-zones', {
         'Belgium',
         'Brazil',
         'Canada',
+        'China',
         'Denmark',
         'Finland',
         'France',
@@ -34,8 +34,8 @@ window.app.component('shipping-zones', {
         'Greece',
         'Hong Kong',
         'Hungary',
-        'Ireland',
         'Indonesia',
+        'Ireland',
         'Israel',
         'Italy',
         'Japan',
@@ -59,10 +59,9 @@ window.app.component('shipping-zones', {
         'Thailand',
         'Turkey',
         'Ukraine',
-        'United Kingdom**',
-        'United States***',
-        'Vietnam',
-        'China'
+        'United Kingdom',
+        'United States',
+        'Vietnam'
       ]
     }
   },