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/restic/snapshots.go

@@ -13,9 +13,11 @@ import (
 // decode only the fields we project to the server; restic's full
 // shape has more (parent, tree, program version) that we don't need.
 //
-// Summary is only populated by restic 0.16+ (which embeds the backup
-// summary inside each snapshot). Older clients leave it nil and the
-// agent reports zero size/file-count — the UI degrades to "—".
+// Summary is only populated by restic 0.17+ (which embeds the backup
+// summary inside each snapshot record). Older clients leave it nil
+// and the agent reports zero size/file-count — the UI degrades to
+// "—" and the column headers carry a tooltip explaining the version
+// requirement (see web/templates/pages/host_detail.html).
 type Snapshot struct {
 	ID       string           `json:"id"`
 	ShortID  string           `json:"short_id"`