Support mouse back/forward buttons for navigation

[lambdalisue]

Summary

macOS mouse side buttons (back/forward) do not trigger navigation in the app. Users expect these buttons to work like in web browsers and other native apps.

Current Behavior

• ✅ Keyboard shortcuts work (Cmd+[ / Cmd+])
• ✅ Header UI buttons work (◀ ▶)
• ❌ Mouse side buttons (button 3/4) do nothing
• ❌ Trackpad two-finger swipe gestures do not work

Technical Analysis

The current implementation only handles left-click (0) and middle-click (1) in the WebView:

// markdown.rs
if (event.button === 0 || event.button === 1) {
    event.preventDefault();
    window.handleMarkdownLinkClick('{escaped_href}', event.button);
}

Mouse back/forward buttons (button: 3 for back, button: 4 for forward) are not captured anywhere in the application.

Proposed Solution

Add a document-level mousedown event listener in the WebView to detect button 3/4 and invoke the existing GoBack/GoForward navigation logic:

1. Inject a global JavaScript listener for mousedown events
2. When event.button === 3 (back) or event.button === 4 (forward), call a Rust handler
3. Reuse the existing HistoryManager navigation methods

Related Files

• desktop/src/history.rs - History management (already implemented)
• desktop/src/components/content/file_viewer.rs - WebView event handling
• desktop/src/menu.rs - Menu handler for GoBack/GoForward
• desktop/src/components/header.rs - UI button handlers

Notes

• Keyboard shortcuts use muda's Accelerator which only supports keyboard Code, not mouse buttons
• Trackpad gestures may require OS-level event handling (separate from this issue)