fix(calendar-sync): guard the hourly timer body against repeating signals

- The sync timer body ran unguarded from an hourly run-at-time timer.
- A signal in the timezone check or the sync fan-out errored every tick.
- The same error, once an hour, indefinitely.
- Wrapped the body in condition-case; an error is logged, not propagated.
- Also demoted the timezone-change message to the silent log, like the
  module's other timer-path notices — an hourly timer shouldn't spam the echo area.

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