fix(version): use internal/version as single source for build constants

The Dockerfile only set `-X main.version=...`, so docker-built binaries
left `internal/version.Version` at its default "dev". The update logic
(host_update.go:61, hosts.go:94, fleet_update.go:101 et al.) compares
against `internal/version.Version`, so a v1.0.0 host always looked
out-of-date to a v1.0.0 server, the chip never cleared, and pressing
"update" re-downloaded the same bundled binary on a loop.

Collapse the two version sources: drop the `var version/commit/date`
locals in cmd/{server,agent}/main.go, route everything through
internal/version (now also carrying Date), and have both the Dockerfile
and the Makefile set the same single set of -X flags. Verified
end-to-end: make build and docker build both emit binaries whose
--version reflects the build VERSION.

cmd/server/main.go

@@ -26,12 +26,7 @@ import (
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/ui"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/ws"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
-)
-
-var (
-	version = "dev"
-	commit  = "none"
-	date    = "unknown"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/version"
 )
 
 func main() {
@@ -47,7 +42,7 @@ func run() error {
 	flag.Parse()
 
 	if *showVersion {
-		fmt.Printf("restic-manager-server %s (commit %s, built %s)\n", version, commit, date)
+		fmt.Printf("restic-manager-server %s (commit %s, built %s)\n", version.Version, version.Commit, version.Date)
 		return nil
 	}
 
@@ -123,7 +118,7 @@ func run() error {
 		NotificationHub: notifHub,
 		UpdateWatcher:   updateWatcher,
 		UI:              renderer,
-		Version:         version,
+		Version:         version.Version,
 		OIDC:            oidcClient,
 		Metrics:         metricsRegistry,
 	}
@@ -177,7 +172,7 @@ func run() error {
 
 	errCh := make(chan error, 1)
 	go func() {
-		slog.Info("server listening", "addr", cfg.Listen, "version", version)
+		slog.Info("server listening", "addr", cfg.Listen, "version", version.Version)
 		errCh <- srv.Start()
 	}()