Fix save button validation not reliably reflecting input state

[biast12]

Description

Validation state from FormInputRow was not reliably reaching the parent Forms.svelte due to Svelte's $: blocks firing during component initialization before on: listeners are guaranteed to be attached.

Fixes:

• Add onMount dispatch in FormInputRow to guarantee initial validation state is sent after mount
• Add hasValidationErrors guard in saveInputs() to block invalid saves regardless of button UI state
• Assign stable _key to new inputs so inputValidationErrors uses a consistent key instead of the loop index, which shifts on reorder/delete

Type of Change

• Bug fix
• New feature
• Breaking change

Testing

• Create new form
• Add an item, fill it out
• Add a checkbox group, leave it blank
• Move it up above the first item
• Fill checkbox group out properly, save button remains greyed out

Checklist

• My code follows the style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works