fix: stop the voice alert panel crashing on its first alert (#1136)

[MOHITKOURAV01]

Description

VoiceAlertManager renders its pending-alert list with queue.map(...), but queue was
never declared in that file and useVoiceSynthesis only ever returned queueLength. The
list sits behind a queueLength === 0 ? ... : ... guard, so the panel looked fine on load
and died the first time anything was queued:

ReferenceError: queue is not defined
    at VoiceAlertManager (src/components/VoiceAlertManager.jsx:209)

There is no error boundary around it, so the whole voice panel went blank on the very
action it exists for. ESLint has been reporting it as no-undef on this file the entire
time.

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Related Issue

Closes #1136

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Type of Change

• Bug Fix
• New Feature
• Documentation
• UI/UX Improvement
• Refactoring
• Performance Improvement
• Accessibility

---

Changes Made

src/hooks/useVoiceSynthesis.js

• Return queue alongside queueLength. That alone fixes the crash.

• Insert by priority rather than appending. Both call sites tag their alert with a
  priority and one of them is explicitly CRITICAL, but addToQueue was
  setQueue(prev => [...prev, alert]) — a plain FIFO append that nothing read the priority
  in. A critical pollution alert waited behind however many routine messages were already
  queued, which is the wrong end of the queue for a feature meant to be listened to rather
  than watched.

  insertByPriority and priorityRank are exported and pure, so the ordering rules are
  testable without rendering. The item at index 0 is left alone while it is being spoken —
  moving it would leave the row the panel marks PLAYING pointing at a different alert.
  Interrupting speech already underway is clearQueue's job, not an insert's.

• Put processQueue in its effect's dependency list. It closes over queue and config,
  and leaving it out was the react-hooks/exhaustive-deps warning on this file. A config
  change mid-queue now re-evaluates against the new config; isProcessingRef still stops
  the effect re-entering an utterance in flight.

• Stop speech when alerts are switched off. Previously the toggle only declined to start
  the next message, so it appeared to do nothing until the current one finished.

• Ignore an alert with no usable message, so a malformed payload cannot add a silent row
  that blocks the queue.

• Don't setVoices after unmount — the voice list resolves asynchronously and can outlive
  the mount.

src/components/VoiceAlertManager.jsx

• Destructure queue from the hook.
• key={item.id ?? idx}, so an alert queued without an id doesn't collide.
• aria-live="polite" on the pending count — this is the accessibility panel, and the
  queue depth changing is exactly the sort of thing a screen reader user should hear.

Tests (new)

• src/hooks/useVoiceSynthesis.test.js — 21 tests: the full priorityRank table including
  case-insensitivity and unknown levels, insertByPriority ordering and immutability, and
  the hook's queue draining, priority ordering, disabled-mid-speech, clear, and
  failed-utterance-doesn't-wedge-the-queue behaviour.
• src/components/VoiceAlertManager.test.jsx — 9 tests covering the empty state, the
  pending row that used to throw, the PLAYING marker, clearing, the unsupported-browser
  path, and the voice dropdown.

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Testing

• Tested locally
• No console errors
• Existing functionality works as expected

$ npx vitest run src/hooks/useVoiceSynthesis.test.js src/components/VoiceAlertManager.test.jsx
 ✓ src/hooks/useVoiceSynthesis.test.js (21 tests)
 ✓ src/components/VoiceAlertManager.test.jsx (9 tests)
 Test Files  2 passed (2)
      Tests  30 passed (30)

I checked the panel tests actually catch the bug rather than just passing: reverting the
two source files and re-running gives ReferenceError: queue is not defined on every one
of them.

npx eslint src/components/VoiceAlertManager.jsx src/hooks/useVoiceSynthesis.js no longer
reports the no-undef. The two remaining label-has-associated-control warnings on that
file are pre-existing and untouched here.

Note on CI

The Lint, Build and Playwright jobs are red on main and on every open PR right now
(3 parse errors in App.jsx, Leaderboard.jsx and NoisePollutionTracker.jsx — that's
#1129). Nothing in this branch touches those files.

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