Centralize shared values into variables.nix

One file (variables.nix) holding every value that was previously
hardcoded and repeated across modules: LAN domain/CIDR, home/tailnet
domains, cross-host references (nix-cache substituter hostname, NFS
server hostname, remote-builder user), PXE/PBS IPs, timezone, and the
primary username.

Wired in via flake.nix's specialArgs (and home-manager's
extraSpecialArgs for the two home.nix files), so any module picks it
up by just adding `vars` to its function arguments — no explicit
import needed. Two hosts (nix-cache, server) now derive their own
networking.hostName from the same variable other hosts use to reach
them, so there's exactly one place to change either identifier.

Purely mechanical: every substituted value matches what was already
there, confirmed by identical toplevel .drv paths for all 17 targets
before and after.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

modules/raspi/mount-data.nix

@@ -1,26 +1,26 @@
-{ ... }:
+{ vars, ... }:
 
 {
-fileSystems."/mnt/raspi" = {
-  device = "raspberrypi.tail13f623.ts.net:/home/raspi/raspi";
-  fsType = "nfs4";
-  options = [
-    "nofail"
-    "_netdev"
-    "noatime"
+  fileSystems."/mnt/raspi" = {
+    device = "raspberrypi.${vars.tailnetDomain}:/home/raspi/raspi";
+    fsType = "nfs4";
+    options = [
+      "nofail"
+      "_netdev"
+      "noatime"
 
-    # Don't mount until first access
-    "x-systemd.automount"
+      # Don't mount until first access
+      "x-systemd.automount"
 
-    # Unmount after 10 min idle
-    "x-systemd.idle-timeout=600"
+      # Unmount after 10 min idle
+      "x-systemd.idle-timeout=600"
 
-    # Give the Pi/Tailscale a little time to appear
-    "x-systemd.device-timeout=10s"
+      # Give the Pi/Tailscale a little time to appear
+      "x-systemd.device-timeout=10s"
 
-    # Explicitly use NFSv4.2 if supported
-    "nfsvers=4.2"
-  ];
-};
+      # Explicitly use NFSv4.2 if supported
+      "nfsvers=4.2"
+    ];
+  };
 
-}
+}