Fix inconsistent dirty state validation between staging and non-staging modes

[Copilot]

The WriteButton component enforced dirty state validation inconsistently: in non-staging mode the button was disabled when no edits existed, but in staging mode it remained enabled and showed a notification on click.

Changes

• Unified button disable logic: Applied !isDirty check uniformly across all modes instead of only non-staging (!mode.staging && !isDirty)
• Updated test expectations: Changed staging mode test from verifying notification behavior to verifying button is disabled when not dirty

The finalizeEntry guard remains as defensive programming despite the button now preventing clicks when not dirty.

// Before: inconsistent between modes
disabled={Object.keys(fieldsWithErrors).length > 0 || (!mode.staging && !isDirty)}

// After: consistent validation
disabled={Object.keys(fieldsWithErrors).length > 0 || !isDirty}

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[karilint]

LGTM