diff --git a/CLAUDE.md b/CLAUDE.md
index 041302f..e587871 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -17,23 +17,57 @@ There are no tests or linting configured.
 
 This is a macOS Electron desktop app for batch-renaming photo files. It follows strict Electron security practices: `contextIsolation: true`, `nodeIntegration: false`.
 
-**Process boundary:** All filesystem access lives exclusively in `src/main.js` (main process). The renderer never touches `fs` directly.
+**Process boundary:** All filesystem access lives exclusively in `src/main/` (main process). The renderer never touches `fs` directly.
 
-**IPC surface** (defined in `main.js`, bridged via `preload.js` as `window.api`):
-- `select-folder` → opens native folder picker, returns path
-- `list-images` → reads directory, returns `{name, path, mtime, size}[]` filtered to image extensions
-- `read-image` → reads file, returns base64 data URL for display
-- `rename-files({folderPath, files, prefix, digits})` → two-phase rename (temp names first, then final names) to avoid collisions when new names overlap old names
-- `reveal-folder` → opens folder in Finder via `shell.openPath`
+### File structure
 
-**Renderer (`src/renderer.js`):** Single-file, no framework. All state is module-level globals:
+```
+src/
+├── main/
+│   ├── index.js          # Main process entry point; creates BrowserWindow, calls registerAll()
+│   └── ipc/
+│       ├── index.js      # registerAll(mainWindow) — delegates to folder and files modules
+│       ├── folder.js     # select-folder, reveal-folder
+│       └── files.js      # list-images, read-image, rename-files
+├── preload/
+│   └── index.js          # Bridges IPC to window.api via contextBridge (isolated context)
+└── renderer/
+    ├── index.html        # App shell (dark theme, CSS variables in :root)
+    ├── style.css         # All CSS
+    ├── renderer.js       # Main renderer logic; imports state and handlers
+    ├── state.js          # Shared mutable state: files[], currentFolder, selectedIdxs, sortMode
+    └── handlers/
+        ├── drag-drop.js  # Card drag-and-drop reorder (single and multi-card)
+        ├── selection.js  # Rubber-band selection, Shift-click, Cmd+click
+        ├── pinch-zoom.js # Trackpad pinch to resize thumbnails (CSS --thumb-size, 150–400px)
+        └── hover-preview.js # Hover preview overlay
+```
+
+### IPC surface
+
+Defined in `src/main/ipc/`, bridged via `src/preload/index.js` as `window.api`:
+
+| Channel | Direction | Description |
+|---|---|---|
+| `select-folder` | invoke | Opens native folder picker, returns path or null |
+| `list-images` | invoke | Reads directory, returns `{name, path, mtime, size}[]` filtered to image extensions, sorted by filename |
+| `read-image` | invoke | Reads file, returns base64 data URL for display |
+| `rename-files({folderPath, files, prefix, digits})` | invoke | Two-phase rename: temp names first, then final names to avoid collisions |
+| `reveal-folder` | invoke | Opens folder in Finder via `shell.openPath` |
+| `fullscreen-change` | push (main→renderer) | Sent on `enter-full-screen` / `leave-full-screen` window events |
+
+### Renderer (`src/renderer/`)
+
+Uses ES module imports (`type="module"`). No framework.
+
+**State** (`src/renderer/state.js`):
 - `files[]` — ordered array of file objects; order determines rename sequence
 - `selectedIdxs` — `Set<number>` of selected card indices
-- Thumbnail grid uses CSS `--thumb-size` custom property (150–400px), adjusted via trackpad pinch (`ctrlKey + wheel`)
-- Rubber-band selection: drag on empty grid area to select multiple cards; `Shift` adds to existing selection; `Cmd+click` toggles individual cards
-- Drag-and-drop reorder: single card drag or multi-card drag (moves selected group); DOM reorder avoids re-fetching images by remapping existing card elements via `card.dataset.filepath`
-- Thumbnails load in batches of 10 (`Promise.all`) to avoid blocking the UI
+- `currentFolder` — active directory path
+- `sortMode` — `'name'` | `'date'` | `null`
 
-**Styling:** All CSS is inline in `src/index.html`. Dark theme with CSS variables defined in `:root`. The macOS traffic-light buttons sit in `#titlebar` which uses `-webkit-app-region: drag`.
+**Thumbnail loading:** Uses `IntersectionObserver` with `rootMargin: '400px 0px'` — images load lazily as cards enter the viewport. DOM card elements are created immediately without `src`; the observer fills in `img.src` on entry. Drag-and-drop reorder remaps existing card elements via `card.dataset.filepath` to avoid re-fetching images.
+
+**Styling:** All CSS lives in `src/renderer/index.html`. Dark theme with CSS variables defined in `:root`. The macOS traffic-light buttons sit in `#titlebar` with `-webkit-app-region: drag`.
 
 **Build output:** `electron-builder` targets DMG for arm64 and x64. The app icon must be at `assets/icon.icns`.
diff --git a/src/main/index.js b/src/main/index.js
index c18f4f7..dfad217 100644
--- a/src/main/index.js
+++ b/src/main/index.js
@@ -13,7 +13,7 @@ function createWindow() {
     titleBarStyle: 'hiddenInset',
     backgroundColor: '#0f0f17',
     webPreferences: {
-      preload: path.join(__dirname, 'preload.js'),
+      preload: path.join(__dirname, '../preload/index.js'),
       contextIsolation: true,
       nodeIntegration: false,
     },
diff --git a/src/main/preload.js b/src/preload/index.js
similarity index 100%
rename from src/main/preload.js
rename to src/preload/index.js