Replace Electron's built-in zoomIn/zoomOut/resetZoom menu roles with
custom implementations that use a 0.1 zoom-level step instead of
Chromium's default 0.2. This makes Ctrl/Cmd + +/-0 zoom feel more
granular and less jumpy.
Also adds installZoomShortcuts() which intercepts the keyboard shortcuts
via before-input-event. This is necessary on Linux/Windows where the
application menu is set to null, so Chromium's default handler would
otherwise apply the full 0.2 step.
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