chore(ci): use absolute KEYCAST_BINARY in e2e workflow

[sergey3bv]

Summary

• Made KEYCAST_BINARY absolute in GitHub workflow.

Avoid naming corporate partners, customers, or other sensitive external brands in this PR title or body. Use generic descriptors unless a maintainer explicitly approves the public reference.

Motivation

• A follow-up for ci: run Playwright e2e OAuth security suite in GitHub checks #173

Related Issue

• Closes chore(ci): make KEYCAST_BINARY absolute in the e2e GitHub Actions workflow #177

Testing

• cargo test --workspace --verbose
• cargo clippy --workspace --all-targets --all-features -- -D warnings -A deprecated
• cargo fmt --all -- --check
• Manual verification completed

Visuals

• UI/web change with screenshots/video attached
• No visual change
• Visuals and text avoid sensitive external brand or partner names unless explicitly approved

[dcadenas]

Approved. This matches the hub-reviewed change for keycast#177: the e2e workflow now uses an absolute KEYCAST_BINARY and reuses it for migrations and the background server.

[dcadenas]

Re-approved after updating the branch with latest main. Checks are passing and the diff still matches keycast#177.