Add shutdown-prep button to dashboard-v2 NodeDetail

Wires safe-shutdown-prep.sh into the dashboard so an operator can
prep a node for hardware maintenance from a browser instead of SSH.

New RPC methods (0x20/0x21):
- ShutdownPrepCheck runs `--dry-run` to completion and returns the
  full report. Never stops anything, safe to call repeatedly.
- ShutdownPrepExecute starts the real run detached (`systemd-run
  --user --scope --collect`), placing it in a cgroup outside
  claw-store.service's own -- the script's own step 6 stops that
  service, i.e. the process that would otherwise be running it, so
  it has to survive its own parent dying. Returns immediately with
  a "started" message; full output lands in
  /var/lib/claw-store/shutdown-prep.log for whoever's at the machine
  once it's gone dark, since there's no way to stream a live result
  past the point the daemon stops itself.
- Execute double-checks confirm_node_name against the peer's own
  configured name server-side, on top of the aggregator's own path
  match -- defense in depth for a highly consequential action.

Aggregator endpoints (admin-token gated, AuthedCaller::require_admin):
  POST /api/v2/node/:name/shutdown-prep/check
  POST /api/v2/node/:name/shutdown-prep/execute

Frontend: ShutdownPrepPanel on NodeDetail. Check button always
enabled; the real "stop services" button only unlocks after a ready
check, and additionally requires typing the exact node name to
confirm before it's clickable.

Also fixes a script bug found while testing this against the live
daemon process (not caught in manual interactive-shell testing): the
zpool-detection line parsed raw `mount` output positionally, which
returned the wrong field under the daemon's process context for
reasons that didn't reproduce interactively. Switched to
`df --output=source`, which is stable across both.

Verified end-to-end against tank, architect, and morpheus, including
cross-node targeting (tank's dashboard successfully triggered a
check on morpheus over the fleet RPC layer).

Co-Authored-By: Claude Sonnet 5 <[email protected]>

dashboard-v2/tsconfig.tsbuildinfo

@@ -1 +1 @@
-{"root":["./src/App.tsx","./src/main.tsx","./src/components/NodeCard.tsx","./src/components/ProjectsPanel.tsx","./src/components/StatTile.tsx","./src/components/StorageBar.tsx","./src/lib/api.ts","./src/pages/CommandCenter.tsx","./src/pages/NodeDetail.tsx","./src/pages/RefTrackingPage.tsx","./src/pages/StorageBrowser.tsx"],"version":"6.0.3"}
+{"root":["./src/App.tsx","./src/main.tsx","./src/components/NodeCard.tsx","./src/components/ProjectsPanel.tsx","./src/components/ShutdownPrepPanel.tsx","./src/components/StatTile.tsx","./src/components/StorageBar.tsx","./src/lib/api.ts","./src/pages/CommandCenter.tsx","./src/pages/NodeDetail.tsx","./src/pages/RefTrackingPage.tsx","./src/pages/StorageBrowser.tsx"],"version":"6.0.3"}