Fixes #3486.

Problem

In Notifications → Battery Alerts, the "Select which nodes to monitor" picker showed a stuck selection count (e.g. "11 node(s) selected") with nothing visibly checked, and "Deselect all" couldn't clear it. Low-battery alerts also silently matched nothing.

Root cause

The picker is source-scoped: it only lists the current source's nodes (/api/nodes?sourceId=…, also channel-permission filtered). The saved selection is per-source too, but node IDs saved under a different source — or for nodes that no longer exist under the active source — stayed in selectedMonitoredNodes while being invisible in the list. "Deselect all" mapped over only the visible/filtered rows (filteredNodes), so those stale IDs were unreachable and the count never went to 0. getLowBatteryMonitoredNodes then found no matching nodes to alert on.

(There is no key-format mismatch — selection and list both compare bare nodeId strings. The IDs are simply filtered out of the source-scoped response.)

Fix

• Reconcile / prune (on load): prune the per-source selection down to IDs that actually resolve under the active source. Guarded on a populated node list so a failed or not-yet-returned fetch can't wipe a valid selection, and works regardless of whether the nodes or the prefs load first (length-guarded, no re-render loop).
• Per-source Select all / Deselect all:
  • No search term → Select all selects the entire current source; Deselect all clears the entire selection (stale IDs included).
  • Search active → previous subset add/remove behaviour preserved, so you can still do targeted multi-select while filtering (and stale IDs outside the filter are left untouched).

Set logic is extracted into pure helpers (src/components/monitoredNodes.ts) with unit tests.

Notes

• For non-admin users, the active-source list is also channel-permission filtered, so reconcile prunes against what the user can actually see/manage in the picker. Admins see the full source list.
• Reconcile updates local state only; the cleaned selection persists when the user saves preferences (consistent with the rest of the form).

Validation

• New unit tests: 10/10 pass (reconcile + both button modes)
• tsc --noEmit: clean
• Full Vitest suite: 6480 pass, 0 fail, 0 suite failures

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