Register the Widevine CDM for Brave (#474)

The widevine package installed by the Apple Silicon migration registers its
CDM for Chromium and Firefox only. Brave, installed through
`omarchy install browser brave`, looks for the CDM inside its own install
directory, finds nothing, and silently fails on DRM sites: Netflix, Apple TV+
and Spotify show a green or blank player with no useful error.

Link the system CDM into /opt/brave-bin at install time, and add a migration
so existing Brave installs are repaired too. Both are no-ops when the widevine
package is absent, which is the case wherever the browser ships its own CDM.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
Co-authored-by: Naeem Malik <malik2015naeem@gmail.com>