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out-of-date to a v1.0.0 server, the chip never cleared, and pressing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.

Makefile

@@ -8,8 +8,10 @@ VERSION        ?= $(shell git describe --tags --always --dirty 2>/dev/null || ec
 COMMIT         ?= $(shell git rev-parse HEAD 2>/dev/null || echo none)
 DATE           ?= $(shell date -u +%Y-%m-%dT%H:%M:%SZ)
 VERSION_PKG    := gitea.dcglab.co.uk/steve/restic-manager/internal/version
-LDFLAGS        := -s -w -X main.version=$(VERSION) -X main.commit=$(COMMIT) -X main.date=$(DATE) \
-                  -X $(VERSION_PKG).Version=$(VERSION) -X $(VERSION_PKG).Commit=$(COMMIT)
+LDFLAGS        := -s -w \
+                  -X $(VERSION_PKG).Version=$(VERSION) \
+                  -X $(VERSION_PKG).Commit=$(COMMIT) \
+                  -X $(VERSION_PKG).Date=$(DATE)
 GOFLAGS        := -trimpath
 DOCKER_IMAGE   ?= gitea.dcglab.co.uk/steve/restic-manager
 DOCKER_TAG     ?= dev