fix(cp): cancel in-flight work when a modal is dismissed

[netravnen]

Dismissing a CP modal hid the work without stopping it. close() was
literally backdrop.remove(), so a backdrop click or Close during an apply
tore down the progress display while the loop kept issuing PUT and DELETE
against live rows -- with no progress shown and no remaining way to
abort, since the Cancel button that sets cancelSignal.cancelled went away
with the modal. The explicit Cancel path had always been correct; close()
simply did not use it.

Two related safeguards were inert for the same reason -- present in the
UI, absent in effect. Each opener resolved at its first render, so the
toolbar handler's action lock was released while the modal was still
open and a second click stacked a second modal, with its own
cancelSignal, over the same rows. And the conflict resolver pre-filled
its type-to-confirm input from first render, so the banner's "type the
ixlan id to enable Apply" gate was satisfied before the admin touched
anything.

Changes:

• close() now sets cancelSignal.cancelled = true before removing the
  backdrop, in the IXLAN renumber, IX-F member audit and conflict
  resolver modals.
• The conflict resolver and recent-IP-changes modals gain a real close()
  and a backdrop-click path; both previously had only an inline
  () => backdrop.remove() on the Close button.
• Each of the four openers now resolves on dismissal rather than on first
  render, so the caller's action lock spans the modal's lifetime. For the
  read-only recent-changes report this is what makes its existing
  "report is already open" message true.
• Drop the conflict resolver's confirmInput.value pre-fill and stop
  showing the expected id as the placeholder -- the placeholder is what
  made an empty field look filled, which is the "false lockout" the
  pre-fill was originally added to work around. The input listeners that
  enable Apply were already wired, so the gate now works as the banner
  describes.
• Give the two fire-and-forget modal launches their own .catch(), per the
  convention added in the pdbFetch commit.

Security:

• Removes a path where an admin who dismissed a modal mid-apply believed
  they had stopped a bulk renumber or merge while PUT/DELETE continued
  against production rows.
• Restores the type-to-confirm gate on the destructive conflict resolver,
  and prevents two concurrent modals from writing to the same ixlan.

Testing:

• node --test: 518 tests, 517 pass, 1 skipped (live tests are opt-in).
• New tests/cp-modal-safeguards.test.js pins all three safeguards. These
  are source assertions, not behavioral ones, and the file says so at
  length: the modals are DOM wiring this repo does not unit test, and the
  shim's FakeElement has no addEventListener, so a modal cannot be
  constructed or clicked through loadScript. They cannot prove the wiring
  works -- the AGENTS.md smoke test covers that -- but they do catch the
  failure that actually happened, which was a safeguard going missing
  with nothing red.
• Each safeguard reverted individually and confirmed red: 4, 10 and 1
  failures respectively.

Backwards Compatibility:

• The four modal openers now settle on dismissal instead of first render.
  All seven call sites are updated here; three already awaited them
  inside the action lock, which is the behavior being corrected.
• No storage, API or audit-log format changes.

Assisted-by: Claude:claude-opus-5

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