Guard storage access for delivery readiness

[love19870821]

Motivation

• Prevent runtime errors in environments where localStorage is unavailable or restricted so the app remains usable.
• Surface a clear, user-facing status when persistent storage cannot be used so users know data will only persist for the current page.

Description

• Add isStorageAvailable() which safely tests localStorage by attempting a set/remove and returns a boolean.
• Guard loadEntries() to return an empty list when storage is unavailable and set speechStatus to a helpful message, and keep loadEntries() JSON parsing inside a try/catch.
• Guard saveEntries() to no-op when storage is unavailable and show a speechStatus message on write failure.

Testing

• Launched a static server with python -m http.server 8000 and ran a Playwright script that opened the page and produced artifacts/voice-ledger-final.png, and the script completed successfully.
• No unit tests were added; the automated UI screenshot run succeeded.

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Codex Task