feat(dashboard): operator removal UI for stale machines/sessions (SPEC-004 Task 5)

Admin-only per-row Remove + multi-select bulk removal on the machines view, plus
per-row purge Remove on the sessions view, wired to the Task-5 admin API
(DELETE /api/machines|sessions/:id?purge=true, POST /api/machines/bulk-remove).
Confirm modals (danger-styled, focus-trapped), TanStack refetch so purged rows
leave the console, structured ApiError surfacing, honest partial-bulk summary,
and admin-gating via useAuth().isAdmin as defense-in-depth over the server 403.
Replaces the legacy all-user delete trigger. typecheck/lint/build clean.

Implements specs/v2-stable-identity/plan.md Task 5 (dashboard portion).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

dashboard/src/components/ui/table.css

@@ -53,6 +53,33 @@
   padding-right: 0 !important;
 }
 
+/* Selection column — fixed narrow rail to the left of the status dot. */
+.dt__select {
+  width: 34px;
+  padding-left: 16px !important;
+  padding-right: 0 !important;
+}
+/* Generous hit target around the checkbox; the label also stops row-click. */
+.dt__checkwrap {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  padding: 6px;
+  margin: -6px;
+  cursor: pointer;
+}
+.dt__check {
+  width: 15px;
+  height: 15px;
+  accent-color: var(--accent);
+  cursor: pointer;
+}
+.dt__check:focus-visible {
+  outline: none;
+  box-shadow: 0 0 0 2px var(--bg), 0 0 0 4px var(--accent-ring);
+  border-radius: 3px;
+}
+
 /* Cell affordances. */
 .dt__mono {
   font-family: var(--font-mono);
@@ -151,3 +178,19 @@
 .toolbar__count .mono {
   color: var(--text);
 }
+
+/* Bulk-action bar: replaces the count readout when rows are selected. */
+.bulkbar {
+  margin-left: auto;
+  display: inline-flex;
+  align-items: center;
+  gap: 10px;
+}
+.bulkbar__count {
+  font-size: 12px;
+  color: var(--text-muted);
+}
+.bulkbar__count .mono {
+  color: var(--text);
+  font-weight: 600;
+}