feat: migrate to react-markdown for robust markdown rendering

- Replace custom markdown parser with react-markdown + remark-gfm
- Fix document link navigation (./ and ../ references)
- Simplify doc viewing flow (direct markdown content instead of parsed structure)
- Update electron main process to only use api folder for docs
- Add blueprint loading from docs/blueprint.md dynamically
- Fix sidebar file selection path matching
- Update preload scripts for new API structure

electron/preload.mjs

@@ -1,9 +1,7 @@
 import { contextBridge, ipcRenderer } from 'electron';
 
 contextBridge.exposeInMainWorld('electronAPI', {
-  selectFolder: () => ipcRenderer.invoke('select-folder'),
-  getDocsPath: () => ipcRenderer.invoke('get-docs-path'),
-  onDocsPathChanged: (callback: (path: string) => void) => {
-    ipcRenderer.on('docs-path-changed', (_event, path) => callback(path));
-  },
+  listDocsFiles: (basePath) => ipcRenderer.invoke('list-docs-files', basePath),
+  readDocFile: (filePath) => ipcRenderer.invoke('read-doc-file', filePath),
+  log: (level, ...args) => ipcRenderer.send('renderer-log', level, ...args),
 });