feat(hosts): the Hosts list labels its figures and says how long; SLA tab gone; one place to add a host (#5279–#5283)

Hosts list:
- CPU and Memory are their own labelled columns (#5279); a silent
  agent's last readings are greyed, and Status says "agent silent for 7m".
- One uptime figure, 30 days, with 24h and 7d on hover. The SLA tab
  (/hosts/uptime) repeated it and is removed (#5280; one home per fact).
- Status is a word with how long ("Down for 12m"); host_status now carries
  down_since. Edit and Delete live on the host's page only (#5281).
- Figures are right-aligned with tabular digits (.table .num), not a
  monospace face.

Agent fleet (#5282): its add-host form is gone, so hosts are added only on
Hosts -> Add host. That form was also the only way to get a token for a
manual curl install, so that moves to the host's Agent card as "Install
manually" (POST /fleet/<host_id>/register), shown on the fleet page as
before. "Delete" there reads "Unregister".

Add host (#5283): laid out like the add-monitor form; a name clash comes
back inline with what was typed instead of a bare text 400; the address
is optional, as the host page already allowed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/hosts/routes.py

@@ -112,7 +112,8 @@ async def create_host():
             try:
                 await host_svc.check_name_free(db, host.name)
             except ValueError as exc:
-                return str(exc), 400
+                return await render_template("hosts/form.html", host=None, values=form,
+                                             error=str(exc)), 400
             db.add(host)
     await log_audit(current_app, session.get("user_id"), session.get("username", ""),
                     "host.created", entity_type="host", entity_id=host.name,
@@ -150,7 +151,8 @@ async def update_host(host_id: str):
                 try:
                     await host_svc.check_name_free(db, name, exclude_id=host.id)
                 except ValueError as exc:
-                    return str(exc), 400
+                    return await render_template("hosts/form.html", host=host, values=form,
+                                                 error=str(exc)), 400
             host.name = name
             host.address = form["address"].strip()
             # Agent and container metrics, and the rules and windows on them,
@@ -296,13 +298,3 @@ async def delete_host(host_id: str):
         await log_audit(current_app, session.get("user_id"), session.get("username", ""),
                         "host.deleted", entity_type="host", entity_id=host_name)
     return redirect(url_for("hosts.list_hosts"))
-
-
-@hosts_bp.get("/uptime")
-@require_role(UserRole.viewer)
-async def uptime_page():
-    async with current_app.db_sessionmaker() as db:
-        result = await db.execute(select(Host).order_by(Host.name))
-        uptime = await host_svc.compute_uptime(db)
-        hosts = host_svc.worst_uptime_first(result.scalars().all(), uptime)
-    return await render_template("hosts/uptime.html", hosts=hosts, uptime=uptime)