Prepare v1.0.0-rc.15 pre-release

[enaboapps]

Prepare v1.0.0-rc.15 from current main, including the merged switch setup, practice, keyboard and settings changes. Synchronize all application and startup-launcher versions and the configuration regression expectation.

Closes #854.

Validation at 14fd9d7:

• Node 24.13.0 and Rust 1.97.1.
• Frontend lint, 217 UI tests, five updater tests and production build passed.
• Rust formatting and Clippy with warnings denied passed; 499 unit tests and seven configuration tests passed. One performance benchmark remains intentionally ignored.
• Startup launcher formatting, Clippy and both tests passed.
• Independent review of this exact head found no actionable findings.
• All CI checks passed, including native Windows and macOS package builds, frontend, dependency audit and CodeQL.

Publication will verify signing, bundled prediction lookup data and the updater feed.

Published v1.0.0-rc.15 through successful release run 35844026908. Verified all seven assets, package checksums, Windows Authenticode signature, prediction lookup bytes in Windows and macOS downloads, and both updater-feed entries. Closed milestone v1.0.0-rc.15 with no open issues.