feat(sidebar): register flowchart panel + rail button + thin fs IPC

- Add data-panel=flowchart rail button in src/index.html after daily-notes

- Append .flowchart-* panel CSS selectors to src/styles-sidebar.css

- Add flowchartIO IPC bridge and registerPanel('flowchart') in src/renderer.js

- Add get-user-data-path / read-text-file / write-text-file IPC in src/main.js

- Path validation: sandbox inside app.getPath('userData')

- read-text-file returns null on ENOENT (file doesn't exist)

Amit Haridas

src/index.html

@@ -2483,6 +2483,32 @@
                 <line x1="3" y1="10" x2="21" y2="10" />
               </svg>
             </button>
+            <!-- Flow Chart Editor: visual Mermaid flowchart builder -->
+            <button
+              class="sidebar-icon"
+              data-panel="flowchart"
+              title="Flow Chart Editor (Ctrl+Alt+F)"
+            >
+              <svg
+                width="20"
+                height="20"
+                viewBox="0 0 24 24"
+                fill="none"
+                stroke="currentColor"
+                stroke-width="2"
+                stroke-linecap="round"
+                stroke-linejoin="round"
+              >
+                <rect x="3" y="3" width="6" height="4" rx="1" />
+                <rect x="15" y="3" width="6" height="4" rx="1" />
+                <polygon points="9,17 12,13 15,17" />
+                <line x1="6" y1="7" x2="6" y2="11" />
+                <line x1="18" y1="7" x2="18" y2="11" />
+                <line x1="6" y1="11" x2="9" y2="15" />
+                <line x1="18" y1="11" x2="15" y2="15" />
+                <line x1="12" y1="17" x2="12" y2="21" />
+              </svg>
+            </button>
           </div>
           <div class="sidebar-panel" id="sidebar-panel">
             <div class="sidebar-panel-header">

src/main.js

@@ -6111,6 +6111,29 @@ ipcMain.handle('read-file', async (event, filePath) => {
   }
   return fs.readFileSync(validation.resolved, 'utf-8');
 });
+ipcMain.handle('get-user-data-path', () => app.getPath('userData'));
+
+ipcMain.handle('read-text-file', async (_event, filePath) => {
+  // Reuse the same path validation as the existing read-file handler.
+  const safe = path.resolve(filePath);
+  if (!safe.startsWith(path.resolve(app.getPath('userData')))) {
+    throw new Error('read-text-file: path outside userData is not allowed');
+  }
+  try {
+    return await fs.promises.readFile(safe, 'utf-8');
+  } catch (err) {
+    if (err.code === 'ENOENT') return null;
+    throw err;
+  }
+});
+
+ipcMain.handle('write-text-file', async (_event, { path: filePath, content }) => {
+  const safe = path.resolve(filePath);
+  if (!safe.startsWith(path.resolve(app.getPath('userData')))) {
+    throw new Error('write-text-file: path outside userData is not allowed');
+  }
+  await fs.promises.writeFile(safe, content, 'utf-8');
+});
 ipcMain.handle('write-file', async (event, payload) => {
   const validation = resolveWritablePath(payload?.path);
   if (!validation.valid) {

src/renderer.js

@@ -154,6 +154,14 @@ if (typeof window !== 'undefined' && !window.electronAPI) {
   };
 }
 
+// Lazy-loaded: filesystem helpers used by the flowchart panel for
+// <userData>/flowchart-session.json auto-save.
+const flowchartIO = {
+  getUserDataPath: () => ipcRenderer.invoke('get-user-data-path'),
+  readFile: (p) => ipcRenderer.invoke('read-text-file', p),
+  writeFile: (p, content) => ipcRenderer.invoke('write-text-file', { path: p, content }),
+};
+
 // Use window.ModalManager if already set by script tag, otherwise require it.
 // This prevents "Identifier 'ModalManager' has already been declared" when
 // both the script tag in index.html and CommonJS require() declare it.
@@ -2406,6 +2414,40 @@ document.addEventListener('DOMContentLoaded', async () => {
       }),
   });
 
+  // Flow Chart Editor panel — visual Mermaid flowchart builder.
+  // Persists to <userData>/flowchart-session.json via injected IPC helpers.
+  // Reuses tabManager.insertAtCursor for the "Insert at Cursor" button.
+  const { renderFlowChartPanel } = require('./sidebar/flowchart-panel');
+  // Reuse the existing Mermaid render path used by the preview pane
+  // (src/renderer.js:1106-1142). Lazily loads mermaid on first use.
+  const renderFlowChartMermaid = (source, targetEl) => {
+    targetEl.innerHTML = '';
+    const div = document.createElement('div');
+    div.className = 'mermaid';
+    div.textContent = source;
+    targetEl.appendChild(div);
+    if (!window.mermaid) {
+      const mermaidModule = require('mermaid');
+      window.mermaid = mermaidModule.default || mermaidModule;
+    }
+    const theme = document.body.className.includes('theme-dark') ? 'dark' : 'default';
+    window.mermaid.initialize({ startOnLoad: false, theme, securityLevel: 'loose' });
+    window.mermaid
+      .run({ nodes: [div] })
+      .catch((err) => console.warn('flowchart preview render failed:', err));
+  };
+  sidebarManager.registerPanel('flowchart', {
+    title: 'Flow Chart',
+    render: (container) =>
+      renderFlowChartPanel(container, {
+        getUserDataPath: flowchartIO.getUserDataPath,
+        readFile: flowchartIO.readFile,
+        writeFile: flowchartIO.writeFile,
+        insertAtCursor: (text) => tabManager.insertAtCursor(text),
+        renderMermaid: renderFlowChartMermaid,
+      }),
+  });
+
   // Wiki-link navigation: open the target note (creating it on demand after
   // confirmation). Delegated on document so it works in every tab's preview.
   document.addEventListener('click', (event) => {

src/styles-sidebar.css

@@ -1023,3 +1023,91 @@ body[class*='dark'] .preview-content a.wiki-link {
   color: var(--text-secondary, inherit);
   opacity: 0.75;
 }
+
+/* === Flow Chart Editor panel === */
+.flowchart-panel {
+  display: flex;
+  flex-direction: column;
+  flex: 1;
+  min-height: 0;
+  outline: none;
+}
+.flowchart-toolbar {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 6px 8px;
+  border-bottom: 1px solid var(--border-color, #444);
+}
+.flowchart-insert-btn {
+  padding: 4px 10px;
+  border: 1px solid var(--accent, #4a9eff);
+  background: var(--accent, #4a9eff);
+  color: var(--accent-fg, #fff);
+  border-radius: 4px;
+  cursor: pointer;
+  font-size: 12px;
+}
+.flowchart-status {
+  margin-left: auto;
+  font-size: 11px;
+  color: var(--text-muted, #888);
+}
+.flowchart-split {
+  display: flex;
+  flex: 1;
+  min-height: 0;
+}
+.flowchart-canvas-host {
+  flex: 7;
+  min-width: 0;
+  position: relative;
+  background: var(--bg-secondary, #1e1e1e);
+}
+.flowchart-canvas-host svg.flowchart-canvas {
+  width: 100%;
+  height: 100%;
+  display: block;
+  cursor: crosshair;
+}
+.flowchart-preview-host {
+  flex: 3;
+  min-width: 0;
+  display: flex;
+  flex-direction: column;
+  border-left: 1px solid var(--border-color, #444);
+}
+.flowchart-preview-source {
+  flex: 1;
+  margin: 0;
+  padding: 8px;
+  font-family: var(--mono-font, monospace);
+  font-size: 11px;
+  overflow: auto;
+  white-space: pre;
+  border-bottom: 1px solid var(--border-color, #444);
+}
+.flowchart-preview-render {
+  flex: 1;
+  padding: 8px;
+  overflow: auto;
+}
+.flowchart-node {
+  cursor: grab;
+}
+.flowchart-node.selected rect,
+.flowchart-node.selected polygon {
+  stroke: var(--accent, #4a9eff);
+  stroke-width: 2;
+}
+.flowchart-edge.selected {
+  stroke: var(--accent, #4a9eff);
+}
+.flowchart-label-input {
+  font-size: 13px;
+  text-align: center;
+  border: 1px solid var(--accent, #4a9eff);
+  background: var(--bg-primary, #fff);
+  color: inherit;
+  z-index: 100;
+}