Keymaps only understood keystrokes, so the mouse back / forward side buttons did nothing. Keymaps now accept mouse shortcuts (Button4, Shift+Button5, …); every preset binds Button4 / Button5 to Navigate Back / Forward. The dispatcher handles side-button presses in the capture phase, so they work over the editor, tool windows and empty space, never change the editor selection and don't trigger the webview's own history navigation. Menus and tooltips keep showing the keyboard shortcut.
Fixes#75
Claude-Session: https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn
