Refactor/dispute reason shared limit

[Unclebaffa]

Summary of Work

1. Shared Logic: Created a shared module disputeReason.ts containing the DISPUTE_REASON_MAX_LENGTH limit and a validation function validateDisputeReason(), covered by unit tests in disputeReason.test.ts.
2. ActionPanel Refactor:
   • Integrated the shared validator in ActionPanel.tsx.
   • Fixed pre-existing undefined references (previousConfirmActionRef, disputeTriggerRef, and isOverLimit) to restore compilation and proper keyboard focus restoration.
   • Updated the visual character counter and screen-reader live region copy format from "X / 500 characters remaining" to "X of 500 characters".
   • Cleaned up unused references to comply with strict ESLint lint rules.
3. Tests & Coverage: Updated component test assertions in ActionPanel.test.tsx. Impacted modules exceed 96% statement coverage (above the 95% minimum requirement).
4. Documentation: Documented the shared validator and counter format in ActionPanel.md.

For a detailed review, please see the walkthrough.md artifact.

Closes #305

[mikewheeleer]

love this — thanks for picking it up 🙌