v0.4.0: redesign tray icon digits as bold seven-segment, add stable download link

The percentage digits on the tray icon were illegible at real tray-icon
size (confirmed via an actual screenshot -- badly enough to read as a
garbled face, not a number), even though the previous dilated-font-plus-
outline design looked fine in this project's own downscaled preview
tests. Replaced with bold seven-segment digits (like a digital clock) in
a single flat color, no separate outline layer -- outline-plus-fill adds
a second set of parallel edges that likely caused the blur in the first
place, and seven-segment shapes are specifically designed for legibility
on low-resolution displays.

Also: CI now also publishes a version-agnostic `claude-usage-widget.exe`
alongside the versioned asset, so releases/latest/download/claude-usage-
widget.exe is a permanent one-click download link that never needs
updating in the README when a new version ships. Added that link
prominently near the top of the README.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>