chore: migrate development environment from Nix flakes to devenv

Replace Nix flake-based development setup with devenv for better
developer experience and more streamlined environment management.

Changes:
  - Remove flake.nix and flake.lock files
  - Add devenv.nix, devenv.yaml, and devenv.lock configuration
  - Update .envrc to use devenv instead of nix develop
  - Remove Docker development setup (compose.dev.yml, docker/mod.just)
  - Expand .gitignore with comprehensive IDE and OS exclusions
  - Remove Docker-related just commands from justfile

devenv.nix

@@ -0,0 +1,36 @@
+{ pkgs, nixpkgs, rust-overlay, ... }:
+let
+  overlays = [ (import rust-overlay) ];
+  system = pkgs.stdenv.system;
+  rustPkgs = import nixpkgs { inherit system overlays; };
+  rustVersion = (rustPkgs.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml);
+in {
+  dotenv.enable = true;
+
+  packages = with rustPkgs; [
+    bacon
+    cargo-deny
+    just
+    postgresql
+    sqls
+    sqlx-cli
+    (rustVersion.override {
+      extensions = [
+        "rust-src"
+        "rustfmt"
+        "clippy"
+        "rust-analyzer"
+      ];
+    })
+  ];
+
+  services.postgres = {
+    enable = true;
+    listen_addresses = "localhost";
+    initialScript = ''
+      CREATE USER georm WITH PASSWORD 'georm' SUPERUSER;
+      CREATE DATABASE georm OWNER georm;
+      GRANT ALL PRIVILEGES ON DATABASE georm TO georm;
+    '';
+  };
+}