[Test Improver] Add unit tests for useInterval composable

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🤖 This is an automated PR from Test Improver, an AI-powered test improvement assistant.

Goal and Rationale

Add tests for shell/composables/useInterval.ts — a lifecycle-aware setInterval wrapper used across the dashboard. This composable had zero test coverage despite containing meaningful behavior:

• timer initialization on onMounted
• cleanup on onBeforeUnmount (preventing memory leaks)
• correct interval delay enforcement

Approach

Tests mount a minimal component that calls useInterval, using jest.useFakeTimers() to control time precisely without flakiness. 6 tests cover:

1. Interval fires at the specified delay after mount
2. Interval fires repeatedly at the correct cadence
3. Interval fires at exact tick boundary (not before)
4. Interval is stopped on unmount — no further calls after cleanup
5. No throw when unmounting before the first tick
6. Multiple independent instances don't interfere with each other

Coverage Impact

Metric	Before	After
Statements	0%	100%
Branches	0%	100%
Functions	0%	100%
Lines	0%	100%

Test Status

✅ All 6 tests pass, no linting warnings.

yarn test:ci shell/composables/__tests__/useInterval.test.ts

Trade-offs

• Low maintenance burden: tests are purely behavioral and not coupled to implementation internals.
• jest.useFakeTimers() / useRealTimers() used per test to avoid timer bleed.

Fixes #

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