ui+store: dashboard polish — repo size projection + header alignment

- Project total_size_bytes onto hosts.repo_size_bytes inside the
  UpsertHostRepoStats transaction. The hosts row column has been
  unwritten since the initial schema in 0001, so the dashboard's
  Repo size cell has always rendered '—' even after backups. Now
  the column updates atomically alongside the host_repo_stats row,
  and FleetSummary's SUM(repo_size_bytes) becomes accurate too.
- Right-align the Alerts column header so it sits over its
  right-aligned value (was floating left of column, ambiguous).
- Add text-ink-mid to the 30d trend / Alerts / Tags headers so all
  column headers share the same brightness.

web/templates/pages/dashboard.html

@@ -213,10 +213,10 @@
           <div><a href="{{index $sortURL "os"}}" class="text-ink-mid hover:text-ink">OS · arch{{if eq $f.Sort "os"}} {{if eq $f.Dir "desc"}}↓{{else}}↑{{end}}{{end}}</a></div>
           <div><a href="{{index $sortURL "last_backup"}}" class="text-ink-mid hover:text-ink">Last backup{{if eq $f.Sort "last_backup"}} {{if eq $f.Dir "desc"}}↓{{else}}↑{{end}}{{end}}</a></div>
           <div class="text-right"><a href="{{index $sortURL "repo_size"}}" class="text-ink-mid hover:text-ink">Repo size{{if eq $f.Sort "repo_size"}} {{if eq $f.Dir "desc"}}↓{{else}}↑{{end}}{{end}}</a></div>
-          <div>30d trend</div>
+          <div class="text-ink-mid">30d trend</div>
           <div class="text-right"><a href="{{index $sortURL "snapshot_count"}}" class="text-ink-mid hover:text-ink">Snapshots{{if eq $f.Sort "snapshot_count"}} {{if eq $f.Dir "desc"}}↓{{else}}↑{{end}}{{end}}</a></div>
-          <div>Alerts</div>
-          <div>Tags</div>
+          <div class="text-ink-mid text-right">Alerts</div>
+          <div class="text-ink-mid">Tags</div>
           <div></div>
         </div>