P2-04: schedule editor UI

Closes the schedule foundations slice — operator can now drive the
plumbing P2-01..03 landed without touching the JSON API.

* New routes:
  - GET  /hosts/{id}/schedules          (list)
  - GET  /hosts/{id}/schedules/new      (create form)
  - POST /hosts/{id}/schedules/new      (create)
  - GET  /hosts/{id}/schedules/{sid}/edit (edit form)
  - POST /hosts/{id}/schedules/{sid}/edit (update)
  - POST /hosts/{id}/schedules/{sid}/delete (delete, confirm-then-redirect)

* List view (web/templates/pages/schedules_list.html):
  status, cron, paths, retention summary, tags, edit/delete buttons.
  Header shows "version N · agent in sync" or "agent at vM" when the
  push hasn't been ack'd yet — backed by host_schedule_version +
  applied_schedule_version. Empty-state CTA points at /schedules/new.

* Create/edit form (web/templates/pages/schedule_edit.html, shared):
  cron expression with five quick-pick presets (daily 3am / every 6h
  / @hourly / weekly Sun / monthly 1st), paths textarea (one per
  line), excludes textarea, tags (comma-separated), retention as six
  numeric fields (mirrors restic's --keep-* flags one-for-one),
  bandwidth caps, enabled toggle. Side panel explains the
  reconciliation flow so the operator knows what saving actually
  does. Validation errors re-render with operator's input intact.

* internal/server/http/ui_schedules.go owns the handlers; reuses
  the same validateSchedule + pushScheduleSetAsync used by the JSON
  API path. Each save audit-logs schedule.created / schedule.updated
  / schedule.deleted (matching the JSON API actions).

* store.RetentionPolicy gains a Summary() method ("last=7, d=14,
  w=4" or "—"). Used by the list view's table cell so templates
  don't have to do any conditional retention rendering.

* Two new template helpers: list (string varargs → []string, used
  for the cron preset row) and joinComma (sibling to joinDot for
  the rare list that wants commas). RetentionPolicy.Summary covers
  the schedule-list case but the helpers are general.

* host_detail.html secondary tabs row converted from inert <div>s
  into <a> links. Snapshots active by default; Schedules now points
  at the new page. Jobs/Repo/Settings remain inert until their
  P2 owners ship.

Hooks UI deferred to P2-15 (lands with the hook execution path).
Single-kind UI (backup only) by design — other kinds get a UI when
their job dispatch lands in P2-05..08.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/server/ui/funcs.go

@@ -32,6 +32,12 @@ func funcMap() template.FuncMap {
 			return *p
 		},
 		"sub": func(a, b int) int { return a - b },
+		// joinComma joins a slice with ", ". Used by the schedule list
+		// to render retention summaries.
+		"joinComma": func(parts []string) string { return strings.Join(parts, ", ") },
+		// list packs strings into a slice — handy for inline ranges
+		// in templates (e.g. quick-pick cron presets).
+		"list": func(items ...string) []string { return items },
 	}
 }