chore: replace once_cell with LazyLock, add clippy to CI, misc cleanup

[xPathin]

Summary

• Replace once_cell::sync::Lazy with std::sync::LazyLock (stable since Rust 1.80), remove once_cell dependency
• Add cargo clippy -- -D warnings to CI and fix existing clippy warnings (collapsible_if)
• Simplify find_unique_path to use timestamp directly instead of looping 1..1000
• Only show "Entries refreshed" toast on manual refresh button click, not after every add/create/update
• Fix delete toast showing desktop file ID instead of display name
• Disable Pango markup on ActionRows to prevent & in app names from being parsed as XML entities

Test plan

• cargo clippy -- -D warnings passes
• cargo test passes (21 tests)
• cargo fmt --check passes
• Manual test: delete toast shows display name
• Manual test: apps with & in name render correctly