feat: notebook per-cell execution via open-terminal REST endpoints

- Add notebook API functions (createNotebookSession, executeNotebookCell, stopNotebookSession)
- Create CellEditor component with CodeMirror for cell editing
- Rewrite NotebookView with session-based execution, Run All, Restart, Stop
- Kernel status indicator with tooltips
- Wire baseUrl/apiKey through FilePreview and FileNav

src/lib/components/chat/FileNav.svelte

@@ -845,6 +845,8 @@
 						const result = await excelToTable(excelWorkbook.Sheets[sheet]);
 						fileOfficeHtml = result.html;
 					}}
+					baseUrl={selectedTerminal?.url ?? ''}
+					apiKey={selectedTerminal?.key ?? ''}
 					{overlay}
 					onSave={async (content) => {
 						const terminal = selectedTerminal;