Remove orphaned key import/generate UI

Phase 3 (auto-provision merchant from account keypair) removed the
generateKeys / importKeys methods and the dialog data fields, but
left the dialog templates and dropdown menu items behind. They
referenced importKeyDialog.show and generateKeyDialog.show, which
were now undefined — breaking the merchant dashboard with
"Cannot read properties of undefined (reading 'show')".

Removes:
- The Import Key and Generate New Key dialogs from index.html
- The corresponding dropdown items from merchant-tab.html
- The 'import-key' and 'generate-key' emits from merchant-tab.js
- The dangling @import-key / @generate-key listeners in index.html

Merchants are auto-provisioned from the account keypair on first
GET; key rotation is handled by the migrate-keys feature instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/components/merchant-tab.js

@@ -19,9 +19,7 @@ window.app.component('merchant-tab', {
     'merchant-deleted',
     'toggle-merchant-state',
     'restart-nostr-connection',
-    'profile-updated',
-    'import-key',
-    'generate-key'
+    'profile-updated'
   ],
   data: function () {
     return {