fix(nix): resolve missing Cargo.toml and Cargo.lock by using upstream niri flake overlay

[Chintanpatel24]

Summary

Fixes #25

Problem

Lniri was refactored into a pure overlay repository, keeping only modified files under src/ and resources/. However, flake.nix still attempted to build using local paths to ./Cargo.toml and ./Cargo.lock (which were removed from this repo), causing nix build to fail with:

error: path '.../Cargo.lock' does not exist

Solution

• Configured upstream Niri (github:niri-wm/niri/v26.04) as a flake input with inputs.nixpkgs.follows = "nixpkgs".
• Applied Lniri's liquid glass and kwin-glass overlay files (src/render_helpers/, niri-config/src/appearance.rs, src/layer/mapped.rs, etc.) directly onto the unpacked upstream source via overrideAttrs in postPatch.
• Updated postInstall to provide both lniri and Lniri symlinks alongside niri, generate shell completions for lniri, and install desktop entry, portal configuration, session launcher, and systemd units with properly patched binary paths.
• Updated flake.lock with locked inputs for niri, nixpkgs, and rust-overlay.
• Verified evaluation across supported systems with nix flake show and nix eval.

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