Add doctor/update commands, dry-run/print-config, and improve icon fallback

README.md

@@ -132,6 +132,7 @@ The table above covers the technical tradeoffs in more detail. In practice, `des
 ### Key Features
 - **System-WebView Runtime Model:** Generated wrappers stay small because Deskify relies on the system WebView instead of bundling a browser runtime per app.
 - **Automatic Icon Fetching:** Scrapes high-quality 128x128 favicons automatically using the Google Favicon API.
+- **Layered Icon Fallbacks:** Tries site-provided icons first (`<link rel="icon">`, `/favicon.ico`), then falls back to the Google Favicon API, then a dummy icon.
 - **XDG-Compliant System Integration:** Safely creates `.desktop` entries in `~/.local/share/applications` and manages application icons in `~/.local/share/icons/hicolor/`.
 - **Wayland/X11 Ready:** Perfectly binds `StartupWMClass` to ensure your DE groups the app exactly to its custom icon (no generic gear icons in GNOME/KDE taskbars).
 - **Kiosk / Fullscreen Mode:** Pin applications perfectly as dashboards.
@@ -232,12 +233,26 @@ deskify build \
   --height 800
 ```
 
+Preview the generated Tauri config without building/installing:
+
+```bash
+deskify build --url "https://chat.com" --name "Chat" --print-config
+```
+
+Preview planned actions only (dry run):
+
+```bash
+deskify build --url "https://chat.com" --name "Chat" --dry-run
+```
+
 * `--icon <PATH>`: Provide a custom PNG icon instead of auto-downloading one.
 * `--fullscreen`: Starts the app in Kiosk mode.
 * `--no-decorations`: Disables native window decorations (frameless window; useful for dashboards/kiosk setups).
 * `--user-agent <UA>`: Useful to bypass webview restrictions on certain platforms.
 * `--dark-mode`: Forces the Tauri webview into a dark theme.
 * `--width <PX>` / `--height <PX>`: Sets the startup resolution.
+* `--print-config`: Prints the generated `tauri.conf.json` and exits.
+* `--dry-run`: Shows planned actions without building/installing.
 
 ### Managing Apps (`list` & `remove`)
 You can view all applications generated by `deskify`:
@@ -259,6 +274,31 @@ deskify remove chatgpt
 
 `remove` expects this **internal ID** (sanitized lowercase letters, numbers, `-`), not the display name.
 
+### Updating Apps (`update`)
+
+Rebuild and reinstall an existing app ID without manually running `remove` + `build`:
+
+```bash
+deskify update chat --url "https://chat.com" --name "Chat" --no-decorations
+```
+
+Alpha note: `update` currently requires `--url` because Deskify does not persist app URLs/config metadata yet.
+
+You can also preview an update:
+
+```bash
+deskify update chat --url "https://chat.com" --dry-run
+deskify update chat --url "https://chat.com" --print-config
+```
+
+### Diagnostics (`doctor`)
+
+Check local prerequisites and common environment issues (Rust/Cargo, `cargo tauri`, `pkg-config`, directories):
+
+```bash
+deskify doctor
+```
+
 ---
 
 ## 🧪 Manual Smoke Test Checklist (Before a Public Release)