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.

web/templates/pages/host_detail.html

@@ -35,8 +35,10 @@
             <div>Snapshot id</div>
             <div>Time</div>
             <div>Paths</div>
-            <div class="text-right">Size</div>
-            <div class="text-right">Files</div>
+            <div class="text-right{{if $page.LegacyRestic}} cursor-help{{end}}"
+                 {{if $page.LegacyRestic}}title="Needs restic 0.17+ on the agent host. This host runs {{$host.ResticVersion}}."{{end}}>Size</div>
+            <div class="text-right{{if $page.LegacyRestic}} cursor-help{{end}}"
+                 {{if $page.LegacyRestic}}title="Needs restic 0.17+ on the agent host. This host runs {{$host.ResticVersion}}."{{end}}>Files</div>
             <div></div>
           </div>