refactor(flake): Consolidate overlay configurations into shared functions

Extract duplicated overlay and home-manager configuration code into two
reusable factory functions:

- mkBaseOverlay: Creates the base overlay with unstable pkgs, custom
  packages, and bitwarden-desktop compatibility. Accepts optional
  unstableOverlays parameter for darwin-specific customizations.

- mkHomeManagerConfig: Creates home-manager configuration with shared
  settings (useGlobalPkgs, useUserPackages, doom-emacs module). Accepts
  sharedModules parameter for platform-specific modules like plasma-manager.

This reduces code duplication across nixosModules, nixosModulesUnstable,
and darwinModules, making the flake easier to maintain and extend.

Implements bead: nixos-configs-ek5

flake.nix

@@ -56,94 +56,75 @@
   };
 
   outputs = { self, nixpkgs, nixpkgs-unstable, nixos-wsl, ... } @ inputs: let
+    # Shared overlay function to reduce duplication across module sets
+    # Parameters:
+    #   unstableOverlays: Additional overlays to apply when importing nixpkgs-unstable
+    mkBaseOverlay = { unstableOverlays ? [] }: (final: prev: {
+      unstable = import nixpkgs-unstable {
+        system = prev.stdenv.hostPlatform.system;
+        config.allowUnfree = true;
+        overlays = unstableOverlays;
+      };
+      custom = prev.callPackage ./packages {};
+      # Compatibility: bitwarden renamed to bitwarden-desktop in unstable
+      bitwarden-desktop = prev.bitwarden-desktop or prev.bitwarden;
+    });
+
+    # Shared home-manager configuration factory
+    # Parameters:
+    #   sharedModules: Additional modules to include in home-manager.sharedModules
+    mkHomeManagerConfig = { sharedModules ? [] }: {
+      home-manager.useGlobalPkgs = true;
+      home-manager.useUserPackages = true;
+      home-manager.sharedModules = sharedModules ++ [
+        inputs.nix-doom-emacs-unstraightened.homeModule
+      ];
+      home-manager.extraSpecialArgs = {
+        globalInputs = inputs;
+      };
+    };
+
     nixosModules = [
       ./roles
-    ] ++ [
       inputs.home-manager.nixosModules.home-manager
       {
-        nixpkgs.overlays = [
-          (final: prev: {
-            unstable = import nixpkgs-unstable {
-              system = prev.stdenv.hostPlatform.system;
-              config.allowUnfree = true;
-            };
-            custom = prev.callPackage ./packages {};
-            # Compatibility: bitwarden renamed to bitwarden-desktop in unstable
-            bitwarden-desktop = prev.bitwarden-desktop or prev.bitwarden;
-          })
-        ];
-        home-manager.useGlobalPkgs = true;
-        home-manager.useUserPackages = true;
-        home-manager.sharedModules = [
-          inputs.plasma-manager.homeModules.plasma-manager
-          inputs.nix-doom-emacs-unstraightened.homeModule
-        ];
-        home-manager.extraSpecialArgs = {
-          globalInputs = inputs;
-        };
+        nixpkgs.overlays = [ (mkBaseOverlay {}) ];
       }
+      (mkHomeManagerConfig {
+        sharedModules = [ inputs.plasma-manager.homeModules.plasma-manager ];
+      })
     ];
+
     # Modules for unstable-based systems (like nix-deck)
     nixosModulesUnstable = [
       ./roles
-    ] ++ [
       inputs.home-manager-unstable.nixosModules.home-manager
       inputs.jovian.nixosModules.jovian
       {
-        nixpkgs.overlays = [
-          (final: prev: {
-            unstable = import nixpkgs-unstable {
-              system = prev.stdenv.hostPlatform.system;
-              config.allowUnfree = true;
-            };
-            custom = prev.callPackage ./packages {};
-            # Compatibility: bitwarden renamed to bitwarden-desktop in unstable
-            bitwarden-desktop = prev.bitwarden-desktop or prev.bitwarden;
-          })
-        ];
-        home-manager.useGlobalPkgs = true;
-        home-manager.useUserPackages = true;
-        home-manager.sharedModules = [
-          inputs.plasma-manager-unstable.homeModules.plasma-manager
-          inputs.nix-doom-emacs-unstraightened.homeModule
-        ];
-        home-manager.extraSpecialArgs = {
-          globalInputs = inputs;
-        };
+        nixpkgs.overlays = [ (mkBaseOverlay {}) ];
       }
+      (mkHomeManagerConfig {
+        sharedModules = [ inputs.plasma-manager-unstable.homeModules.plasma-manager ];
+      })
     ];
+
     darwinModules = [
       ./roles/darwin.nix
-    ] ++ [
       inputs.home-manager.darwinModules.home-manager
       {
         nixpkgs.overlays = [
-          (final: prev: {
-            unstable = import nixpkgs-unstable {
-              system = prev.stdenv.hostPlatform.system;
-              config.allowUnfree = true;
-              overlays = [
-                # Override claude-code in unstable to use our custom GCS-based build
-                # (needed for corporate networks that block npm registry)
-                (ufinal: uprev: {
-                  claude-code = prev.custom.claude-code or (prev.callPackage ./packages {}).claude-code;
-                })
-              ];
-            };
-            custom = prev.callPackage ./packages {};
-            # Compatibility: bitwarden renamed to bitwarden-desktop in unstable
-            bitwarden-desktop = prev.bitwarden-desktop or prev.bitwarden;
+          (mkBaseOverlay {
+            # Override claude-code in unstable to use our custom GCS-based build
+            # (needed for corporate networks that block npm registry)
+            unstableOverlays = [
+              (ufinal: uprev: {
+                claude-code = uprev.callPackage ./packages/claude-code {};
+              })
+            ];
           })
         ];
-        home-manager.useGlobalPkgs = true;
-        home-manager.useUserPackages = true;
-        home-manager.sharedModules = [
-          inputs.nix-doom-emacs-unstraightened.homeModule
-        ];
-        home-manager.extraSpecialArgs = {
-          globalInputs = inputs;
-        };
       }
+      (mkHomeManagerConfig { sharedModules = []; })
     ];
 
   in {