refactor(installer-ui): split app rendering helpers

[peters]

Purpose

• split the installer UI app module into focused theme, widget, icon, and screen helpers
• keep the public app surface stable for main.rs while reducing the root to orchestration
• remove the installer UI entry from the maintainability baseline

Behavior Impact

• no intended user-visible behavior change
• theme configuration, icon loading, install progress updates, and screen rendering keep their existing behavior

Test Evidence

• cargo check -p surge-installer-ui
• cargo test -p surge-installer-ui
• cargo clippy -p surge-installer-ui --all-targets --all-features -- -D warnings -W clippy::pedantic
• ./scripts/check-maintainability.sh
• ./scripts/sync-surge-core-vendor.sh --check
• ./scripts/check-version-sync.sh
• cargo fmt --all -- --check
• RUSTFLAGS="-D warnings" cargo test --workspace
• cargo clippy --all-targets --all-features -- -D warnings
• cargo clippy --workspace --lib --bins --examples -- -D warnings -D clippy::unwrap_used -D clippy::expect_used
• cargo clippy --workspace --all-targets --all-features -- -D warnings -W clippy::pedantic
• dotnet format dotnet/Surge.slnx --verify-no-changes
• dotnet test dotnet/Surge.slnx --configuration Release

Migration Notes

• none