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/CellEditor.svelte

@@ -0,0 +1,109 @@
+<script lang="ts">
+	import '$lib/utils/codemirror';
+	import { basicSetup, EditorView } from 'codemirror';
+	import { keymap } from '@codemirror/view';
+	import { Compartment, EditorState, Prec } from '@codemirror/state';
+	import { indentWithTab } from '@codemirror/commands';
+	import { indentUnit } from '@codemirror/language';
+	import { languages } from '@codemirror/language-data';
+	import { oneDark } from '@codemirror/theme-one-dark';
+	import { onMount, onDestroy, createEventDispatcher } from 'svelte';
+
+	const dispatch = createEventDispatcher();
+
+	export let value = '';
+	export let lang = 'python';
+
+	let container: HTMLDivElement;
+	let editor: EditorView | null = null;
+	let editorTheme = new Compartment();
+	let editorLanguage = new Compartment();
+
+	const getLang = async () => {
+		const language = languages.find((l) => l.alias.includes(lang));
+		return await language?.load();
+	};
+
+	onMount(async () => {
+		const isDark = document.documentElement.classList.contains('dark');
+
+		const extensions = [
+			Prec.highest(keymap.of([
+				{
+					key: 'Mod-Enter',
+					run: () => {
+						dispatch('run');
+						return true;
+					}
+				},
+				{
+					key: 'Escape',
+					run: () => {
+						dispatch('cancel');
+						return true;
+					}
+				}
+			])),
+			basicSetup,
+			keymap.of([indentWithTab]),
+			indentUnit.of('    '),
+			EditorView.updateListener.of((e) => {
+				if (e.docChanged) {
+					value = e.state.doc.toString();
+					dispatch('change', value);
+				}
+			}),
+			editorTheme.of(isDark ? oneDark : []),
+			editorLanguage.of([]),
+			EditorView.theme({
+				'&': { fontSize: '0.75rem' },
+				'.cm-content': { padding: '0.35rem 0', fontFamily: 'ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace' },
+				'.cm-gutters': { display: 'none' },
+				'.cm-focused': { outline: 'none' },
+				'.cm-scroller': { overflow: 'auto' }
+			})
+		];
+
+		editor = new EditorView({
+			state: EditorState.create({ doc: value, extensions }),
+			parent: container
+		});
+
+		const language = await getLang();
+		if (language && editor) {
+			editor.dispatch({ effects: editorLanguage.reconfigure(language) });
+		}
+
+		// Watch dark mode
+		const observer = new MutationObserver(() => {
+			const dark = document.documentElement.classList.contains('dark');
+			editor?.dispatch({ effects: editorTheme.reconfigure(dark ? oneDark : []) });
+		});
+		observer.observe(document.documentElement, { attributes: true, attributeFilter: ['class'] });
+
+		editor.focus();
+
+		return () => {
+			observer.disconnect();
+			editor?.destroy();
+			editor = null;
+		};
+	});
+
+	onDestroy(() => {
+		editor?.destroy();
+		editor = null;
+	});
+</script>
+
+<div bind:this={container} class="nb-cm-editor" />
+
+<style>
+	.nb-cm-editor {
+		width: 100%;
+		background: #fffef5;
+	}
+	:global(.dark) .nb-cm-editor {
+		background: transparent;
+	}
+</style>