Adds theme presets (Toffee/Slate/Evergreen/Lavender) with a "follow system"
mode and high-contrast awareness, a first-run welcome screen, and the
Windows analog of Android's share sheet for a Win32 app: an aerofetch://
protocol handler plus an Explorer "Send to AeroFetch" entry, both routed
through a single-instance lock so a second launch hands its link to the
already-running window instead of opening a duplicate.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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