Rename macOS app branding to Rsnap

[yvette-carlisle]

Summary:

• Rename the visible macOS product branding from rsnap to Rsnap and stage/install the app as Rsnap.app.
• Add a canonical app identity spec while preserving stable lowercase technical identifiers such as crate names, bundle id, env vars, and build directory names.
• Update Screen Recording onboarding to show Rsnap.app, center the drag chip content, and remove the duplicate lower Screen Recording status row.

Validation:

• cargo make fmt
• cargo make fmt-check
• cargo test -p rsnap staged_native_host_bundle_uses_product_case
• cargo make lint-swift
• cargo make test-macos-native-host-stage
• cargo make lint-rust
• cargo make test-rust
• git diff --check
• codesign --verify --deep --strict /Applications/Rsnap.app