docs: document brew trust step for Homebrew 6.0+

[eboudrant]

Summary

Homebrew 6.0 (June 2026) requires third-party taps to be trusted before it will load their casks. Install and upgrade now fail with:

Error: Refusing to load cask eboudrant/tap/papastudio from untrusted tap eboudrant/tap.

This documents the one-time brew trust eboudrant/tap step in three places:

• README — added to the Install block (marked 6.0+ only) and an Update note pointing at brew trust when the upgrade hits the error.
• docs/index.html (Pages site) — added to the install <pre> block with a caption explaining the 6.0+ requirement.
• static/js/home.js — chained brew trust eboudrant/tap && into the in-app update banner's copy command. brew trust is idempotent, so it stays a no-op once trusted.

Test plan

• home.js change only affects the update banner's command string. The banner renders only when updateCheck.checkForUpdate() returns available: true, which requires PAPASTUD_VERSION to be set (src/updateCheck.js:75). The Playwright test server doesn't set it, so the banner never renders under test and no screenshot baseline changes. Docker screenshot run was blocked locally by a Docker Desktop corp sign-in; relying on CI's Screenshot Tests to confirm.
• README and docs/index.html are not covered by automated tests (docs-only).