refactor: migration architecture clean (Rust + JS)

Côté JS : décompose main.js en modules ES6 (state, services, ui, router)
pour séparer les responsabilités et faciliter la maintenance.
Côté Rust : ajoute 7 tests sur is_relevant_path (notify_watcher)
pour couvrir les cas cachés/non-md ; couverture totale 77 % > seuil 60 %.

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

src/router.js

@@ -0,0 +1,74 @@
+import { appState } from './state/app-state.js';
+import { listMdFiles } from './services/files.js';
+import { renderSidebar } from './ui/sidebar.js';
+import { loadPage } from './ui/reader.js';
+import { saveRecent, renderRecents } from './ui/home.js';
+
+const viewHome = document.getElementById('view-home');
+const viewReader = document.getElementById('view-reader');
+const sidebarEl = document.querySelector('#sidebar');
+const content = document.querySelector('#content');
+const win = window.__TAURI__.window.getCurrentWindow();
+const titlebarTitle = document.getElementById('titlebar-title');
+
+export function showHome() {
+  appState.currentPath = null;
+  appState.currentMode = null;
+  sidebarEl.innerHTML = '';
+  win.setTitle('Pena — Markdown Viewer');
+  titlebarTitle.textContent = 'Pena — Markdown Viewer';
+  viewReader.classList.add('hidden');
+  viewHome.classList.remove('hidden');
+  renderRecents(openPath);
+}
+
+export function showReader() {
+  viewHome.classList.add('hidden');
+  viewReader.classList.remove('hidden');
+}
+
+export async function openPath(path, mode) {
+  appState.currentPath = path;
+  appState.currentMode = mode;
+  saveRecent(path, mode);
+
+  if (mode === 'file') {
+    showReader();
+    const sep = path.includes('\\') ? '\\' : '/';
+    const parentDir = path.split(sep).slice(0, -1).join(sep);
+    try {
+      const files = await listMdFiles(parentDir);
+      sidebarEl.classList.remove('hidden');
+      content.style.marginLeft = '';
+      renderSidebar(parentDir, files, path, filePath => loadPage(filePath, appState.currentMode, sidebarEl));
+    } catch {
+      sidebarEl.innerHTML = '';
+      sidebarEl.classList.add('hidden');
+      content.style.marginLeft = '0';
+    }
+    await loadPage(path, appState.currentMode, sidebarEl);
+    return;
+  }
+
+  // mode === 'dir'
+  sidebarEl.classList.remove('hidden');
+  content.style.marginLeft = '';
+
+  try {
+    const files = await listMdFiles(path);
+    showReader();
+
+    const sep = path.includes('\\') ? '\\' : '/';
+    const home = files.find(f => {
+      const name = f.split(sep).pop();
+      return name === 'Home.md' || name === 'home.md';
+    });
+
+    const firstFile = home ?? files[0];
+    renderSidebar(path, files, firstFile, filePath => loadPage(filePath, appState.currentMode, sidebarEl));
+    await loadPage(firstFile, appState.currentMode, sidebarEl);
+  } catch (err) {
+    showReader();
+    content.innerHTML = `<p class="error">Impossible d'ouvrir le dossier : ${err}</p>`;
+  }
+}