fix: stop startup when an existing settings file cannot be read

[martin-forge]

Problem

When an existing settings file temporarily reads as null, TaskNotes blocks settings writes but can still start services using defaults. That can start background work with the wrong configuration.

Solution

Stop loading the plugin after the existing recovery reads fail. Leave the unreadable file untouched so it can be restored. A new install with no settings file still starts normally. Follow-up to #1591.

Testing

• All three settings-loss regression tests pass, including new-install behavior.
• npm run lint and npm run build:test pass.
• Reloaded the standalone build in a disposable Obsidian vault; no captured errors.
• The same startup guard passed a live null-settings check: no task, Calendar or timer services started and the damaged bytes stayed unchanged.
• GitHub CI: full test suite and build passed.