fix(web): use i18n translation for hardcoded "Create Session" strings (#259)

Replace hardcoded English strings with translation function calls
in SpawnSession.tsx and router.tsx. Add newSession.title key for
the NewSessionPage header to match the newSession.* namespace.

web/src/lib/locales/en.ts

@@ -91,6 +91,7 @@ export default {
   'button.paste': 'Paste',
 
   // New session form
+  'newSession.title': 'Create Session',
   'newSession.machine': 'Machine',
   'newSession.directory': 'Directory',
   'newSession.placeholder': '/path/to/project',