fix(client): recover SSE on browser visibilitychange

[dimakis]

Summary

• iOS Safari kills EventSource connections when the tab is backgrounded without firing an error event
• The Capacitor lifecycle hook already handled this, but browser clients (e.g. phone via Safari PWA) had no recovery
• This caused the health SSE stream to die permanently after a single background cycle, hiding the voice mic button

Fix

Add a visibilitychange listener in event-bus-singleton.ts that calls ensureConnected() when the page becomes visible.

Test plan

• New test: event-bus-singleton.test.ts — verifies ensureConnected called on visible, not called on hidden
• Full test suite passes (5 pre-existing failures unrelated to this change)
• Manual: open Mitzo in mobile Safari, background the tab, return — mic button should persist

🤖 Generated with Claude Code

[dimakis]

Centaur Review

Found 3 issue(s) (1 critical) (1 warning).

frontend/src/lib/event-bus-singleton.ts

The PR ships tests for a visibilitychange recovery feature whose implementation is missing from the diff — the singleton module has no listener, so both tests will fail.

• 🔴 bugs: The PR adds tests for a visibilitychange listener that calls eventBus.ensureConnected(), but the listener itself was never added to event-bus-singleton.ts. The module still only exports the EventBus and calls connect() — there is no document.addEventListener('visibilitychange', ...). Both tests will fail because dispatching the event does nothing. The commit message says 'Add a visibilitychange listener in the EventBus singleton' but the implementation is missing from the diff. [fixable]

frontend/src/lib/__tests__/event-bus-singleton.test.ts

The PR ships tests for a visibilitychange recovery feature whose implementation is missing from the diff — the singleton module has no listener, so both tests will fail.

• 🔵 missing_tests: No test verifies the actual recovery behavior — that ensureConnected() recreates a CLOSED EventSource. The tests only check that ensureConnected is called, but a test confirming the EventSource is recreated when readyState is CLOSED (the real bug scenario from the commit message) would add meaningful coverage beyond the spy check. [fixable]
• 🟡 unsafe_assumptions (L30): The test mutates document.visibilityState via Object.defineProperty without restoring it in an afterEach. Since both tests modify the same global, a test ordering dependency exists — the second test ('hidden') could inadvertently pass if it relies on state left by the first test. Use afterEach to restore the original descriptor. [fixable]

[dimakis]

🟡 unsafe_assumptions: The test mutates document.visibilityState via Object.defineProperty without restoring it in an afterEach. Since both tests modify the same global, a test ordering dependency exists — the second test ('hidden') could inadvertently pass if it relies on state left by the first test. Use afterEach to restore the original descriptor. [fixable]

[dimakis]

Centaur Review

LGTM — no issues found.