fix: mobile nav full height + upload paste box, PDF zoom fit

- Mobile nav sheet: flex layout, bottom section pushed to bottom
- Upload paste box + file picker added to mobile nav drawer
- Date shown at bottom of mobile nav
- PDF viewer: added #zoom=page-fit to iframe URL

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

frontend-v2/src/lib/pages/brain/AtelierBrainPage.svelte

@@ -583,7 +583,7 @@
           {@const pdfAsset = selectedItem.assets?.find(a => a.asset_type === 'original_upload')}
           {#if pdfAsset}
             <iframe
-              src="/api/brain/storage/{selectedItem.id}/original_upload/{pdfAsset.filename}"
+              src="/api/brain/storage/{selectedItem.id}/original_upload/{pdfAsset.filename}#zoom=page-fit"
               title={selectedItem.title || 'PDF'}
               class="viewer-iframe"
             ></iframe>