feat(frontend): close bottom panel when entering annotate mode

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/documents/[id]/+page.svelte

@@ -66,6 +66,11 @@ $effect(() => {
 	}
 });
 
+// Close the panel when entering annotate mode so the PDF is fully visible.
+$effect(() => {
+	if (annotateMode) panelOpen = false;
+});
+
 // ── Bottom panel state ────────────────────────────────────────────────────────
 
 const LS_KEY_OPEN = 'doc-panel-open';