test: add holdings page sell balance update coverage (#574)

[extolkom]

Summary

Closes #574

Adds integration test coverage for the holdings page reflecting the
correct balance after a successful key sell — covering both partial
sells (quantity decrements) and full sells (holding entry removed
entirely), with the update happening reactively rather than via a
manual refresh.

What's included

Tests added ([test file path])

• decrements_holding_quantity_after_partial_sell_confirmation —
  mocks a holding of 3 keys, simulates a sell of 1 confirming
  successfully, asserts the displayed quantity updates to 2
• removes_holding_entry_after_full_sell_confirmation — mocks a
  holding of 1 key, simulates a full sell, asserts the holding entry
  is no longer rendered
• updates_holdings_without_manual_refresh — asserts the quantity
  update reflects via the app's existing reactive state/query
  invalidation, without any forced refetch or re-render in the test
• partial_sell_leaves_other_holdings_unaffected — mocks two holdings
  for two different creators, sells from one, asserts the other is
  unchanged

Scope / non-goals

• No changes to the holdings page component or sell-confirmation flow
  itself — this PR is test coverage only, per the issue scope
• [If step 5's stop condition triggered: note here that a specific
  test failed against current main and why, and that component logic
  was NOT modified to force it to pass — flag this clearly as a
  possible pre-existing regression for maintainer triage]

Acceptance criteria (from #574)

• Holding quantity decrements correctly after sell confirmation
• Holding entry removed when quantity reaches 0
• Holdings page does not require a manual refresh to reflect the change
• Test covers both partial sell and full sell cases

Verification

• npm test -- holdings — [pass/fail summary]
• npm run typecheck — [pass/fail]
• npm run lint — [pass/fail]
• Branch rebased on latest origin/main

Notes for reviewers

[Fill in if anything surprising came up — e.g. if the "no manual
refresh" test required digging into how the app invalidates queries,
or if any existing test conventions had to be inferred rather than
matched exactly]

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