Improve metadata prompts; fix upload error + remove debug log

- Replace bracketed metadata placeholders with prompting questions
- Fix upload error message (response.statusContents -> statusText)
- Remove stray debug console.log on reactive screen updates
- Document the upload token's security model in the app README

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

bicorder-app/src/App.svelte

@@ -77,11 +77,6 @@
   $: currentScreenData = screens[currentScreen];
   $: totalScreens = screens.length;
 
-  // Debug: log when screens change
-  $: if (screens) {
-    console.log(`Screens updated: ${screens.length} total, shortform: ${data.metadata.shortform}`);
-  }
-
   function goToNextScreen() {
     if (currentScreen < totalScreens - 1) {
       currentScreen++;