feat: show live external URL preview instead of slug on create page

As the user types the app name, the URL builds in real-time:
  /{envSlug}/{appSlug}/

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

ui/src/pages/AppsTab/AppsTab.tsx

@@ -281,8 +281,8 @@ function CreateAppView({ environments, selectedEnv }: { environments: Environmen
         <span className={styles.configLabel}>Application Name</span>
         <Input value={name} onChange={(e) => setName(e.target.value)} placeholder="e.g. Payment Gateway" disabled={busy} />
 
-        <span className={styles.configLabel}>Slug <span className={styles.configHint}>(auto-generated)</span></span>
-        <Input value={slug} onChange={(e) => { setSlug(e.target.value); setSlugEdited(true); }} disabled={busy} />
+        <span className={styles.configLabel}>External URL</span>
+        <MonoText size="sm">/{env?.slug ?? '...'}/{slug || '...'}/</MonoText>
 
         <span className={styles.configLabel}>Environment</span>
         <Select value={envId} onChange={(e) => setEnvId(e.target.value)} disabled={busy}