ui: snapshots SIZE/FILES tooltip when host's restic is < 0.17

Per-snapshot size + file-count come from the embedded summary block
restic added to 'snapshots --json' in 0.17 (the source comment in
internal/restic/snapshots.go incorrectly said 0.16+). Hosts running
0.16.x leave those columns blank.

- Fix the snapshots.go doc comment: '0.16+' -> '0.17+'.
- hostDetailPage carries a LegacyRestic bool computed from the host's
  reported ResticVersion via Env.AtLeastVersion(0, 17). Empty version
  also counts as legacy (conservative default).
- Template attaches title='Needs restic 0.17+ on the agent host. This
  host runs <ver>.' + cursor:help on the SIZE / FILES headers when
  the flag is true. Hosts already on 0.17+ get no tooltip and no
  extra styling.

A host upgrading restic to 0.17+ gets the columns populated on the
next backup automatically — no further code change needed.

internal/server/http/ui_handlers.go

@@ -16,6 +16,7 @@ import (
 
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/api"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/auth"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/restic"
 	"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"
@@ -569,6 +570,12 @@ type hostDetailPage struct {
 	// SnapshotsShown is the number rendered (we cap at ~50 for the
 	// first slice; pagination lands when it matters).
 	SnapshotsShown int
+	// LegacyRestic is true when the host's restic version predates
+	// 0.17, in which case `restic snapshots --json` doesn't embed the
+	// per-snapshot summary block and the Size/Files columns render
+	// blank. The template uses this to attach a tooltip to those
+	// column headers explaining the version requirement.
+	LegacyRestic bool
 }
 
 // handleUIHostDetail is the host detail page (snapshots tab by default).
@@ -611,6 +618,7 @@ func (s *Server) handleUIHostDetail(w stdhttp.ResponseWriter, r *stdhttp.Request
 		hostChromeData: s.loadHostChrome(r, *host, "snapshots", "snapshots"),
 		Snapshots:      shown,
 		SnapshotsShown: len(shown),
+		LegacyRestic:   !restic.Env{Version: host.ResticVersion}.AtLeastVersion(0, 17),
 	}
 	if err := s.deps.UI.Render(w, "host_detail", view); err != nil {
 		slog.Error("ui: render host_detail", "err", err)