system: replace host detection with capabilities module

Replace scattered osConfig.system.name comparisons with a declarative
host.capabilities module. This improves maintainability and semantic
clarity by expressing what capabilities a host has rather than checking
its name.

Changes:
- Add system/host-capabilities.nix with options for hardware, form factor,
  UI behavior, services, and backup configuration
- Configure capabilities in hosts/gunter and hosts/magicman
- Migrate 6 files to use capabilities: packages, waybar, ssh, backup-home
- Remove redundant host name check for pciutils in gunter config
- Make backup-home service conditionally enabled based on capabilities

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

system/host-capabilities.nix

@@ -0,0 +1,64 @@
+{ lib, ... }:
+with lib;
+{
+  options.host.capabilities = {
+    # Hardware capabilities
+    hasCuda = mkOption {
+      type = types.bool;
+      default = false;
+      description = "Whether the host has CUDA-capable GPU (for btop, OBS, etc.)";
+    };
+
+    hasBattery = mkOption {
+      type = types.bool;
+      default = false;
+      description = "Whether the host has a battery (laptop)";
+    };
+
+    # Form factor
+    formFactor = mkOption {
+      type = types.enum [
+        "desktop"
+        "laptop"
+      ];
+      default = "desktop";
+      description = "Physical form factor of the host";
+    };
+
+    # UI behavior customizations
+    volumeScrollStep = mkOption {
+      type = types.int;
+      default = 5;
+      description = "Volume adjustment step percentage for scroll wheel";
+    };
+
+    # Service-specific features
+    enableArrhist = mkOption {
+      type = types.bool;
+      default = false;
+      description = "Enable Sonarr/Radarr monitoring widget (arrhist)";
+    };
+
+    # Network environment features
+    hasEduroamAccess = mkOption {
+      type = types.bool;
+      default = false;
+      description = "Whether this host can connect to eduroam (for SSH config)";
+    };
+
+    # Backup configuration
+    backupRepository = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = "Restic backup repository URL for this host";
+      example = "rest:http://10.69.12.52:8000/gunter.home.2rjus.net";
+    };
+
+    backupPassword = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = "Restic backup password identifier for this host";
+      example = "gunter.home.2rjus.net";
+    };
+  };
+}