docs: add INSTALL.md and update project documentation for release pipeline

Co-authored-by: Junie <junie@jetbrains.com>

INSTALL.md

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+# Installing WoW Backup
+
+This guide provides step-by-step instructions for installing and running **WoW Backup** on macOS, Windows, and Linux.
+
+---
+
+## Prerequisites
+
+WoW Backup packaged distributions include a bundled Java runtime environment (JRE). You **do not** need to install Java or any external dependencies to run the application.
+
+---
+
+## Download
+
+Download the appropriate installer for your platform from the **[GitHub Releases](https://github.com/rukira/wow-backup/releases)** page:
+
+| Platform | Package Format | Installer File Pattern |
+|---|---|---|
+| **macOS** | DMG Disk Image | `WoW Backup-<version>.dmg` |
+| **Windows** | Windows Installer (MSI) | `WoW Backup-<version>.msi` |
+| **Linux** | Debian Package (DEB) | `wow-backup_<version>_amd64.deb` |
+
+---
+
+## macOS Installation
+
+1. Download the `.dmg` file from the latest release.
+2. Double-click the `.dmg` file to mount the disk image.
+3. Drag **WoW Backup.app** into your `/Applications` folder.
+4. Eject the disk image.
+
+### Unsigned Application / Gatekeeper Notice
+Because WoW Backup is distributed without Apple Developer ID code-signing notarization, macOS Gatekeeper may show a warning on first launch (*"WoW Backup cannot be opened because the developer cannot be verified"* or *"Apple could not verify that WoW Backup is free of malware"*).
+
+To launch the app for the first time:
+
+- **Option A (Finder)**:
+  1. Open Finder and navigate to `/Applications`.
+  2. Right-click (or Control-click) on **WoW Backup.app** and choose **Open**.
+  3. In the security popup, click **Open**.
+- **Option B (Terminal)**:
+  Run the following command in Terminal to clear the quarantine attribute:
+  ```shell
+  xattr -d com.apple.quarantine "/Applications/WoW Backup.app"
+  ```
+
+---
+
+## Windows Installation
+
+1. Download the `.msi` installer from the latest release.
+2. Double-click the `.msi` file to start the installation wizard.
+3. Follow the on-screen prompts to complete installation.
+4. Launch **WoW Backup** from the Start Menu or desktop shortcut.
+
+### Windows Defender SmartScreen
+If Windows Defender SmartScreen flags the installer (*"Windows protected your PC — Microsoft Defender SmartScreen prevented an unrecognized app from starting"*):
+1. Click **More info**.
+2. Click **Run anyway**.
+
+---
+
+## Linux Installation
+
+1. Download the `.deb` package from the latest release.
+2. Open your terminal in the download folder and install the package:
+   ```shell
+   sudo apt install ./wow-backup_*.deb
+   ```
+   *Alternatively, with `dpkg`:*
+   ```shell
+   sudo dpkg -i wow-backup_*.deb
+   sudo apt-get install -f
+   ```
+3. Launch **WoW Backup** from your desktop application launcher or run `wowbackup` in the terminal.
+
+---
+
+## First-Time Setup & Overview
+
+Once started, WoW Backup runs discreetly in the system tray / menu bar:
+
+1. **Accessing Dashboard**: Click the tray icon in the macOS menu bar or Windows/Linux system tray to open the application window.
+2. **Settings Configuration**:
+   - Navigate to **Settings** (gear icon).
+   - Set your **World of Warcraft Installation Folder** (e.g. `_retail_` or `_classic_`).
+   - Select your **Backup Destination Folder**.
+   - Set your preferred **Daily Backup Time**, **Retention Policy** (number of backups to keep), and **Compression Mode** (`ZIP` or folder replication).
+   - Optionally enable **Launch at Startup** so backups occur automatically without manual intervention.
+3. **Lifecycle**: Closing the main window hides it to the tray while keeping the scheduler running. To quit the application completely, select **Quit** from the tray menu.