fix(components): align useResponsePanelCopy with onShow* naming

[saagpatel]

What

Brings `useResponsePanelCopy` onto the same contract as the other 12 Wave 5.7 hooks:

1. Rename `showSuccess`/`showError` params to `onShowSuccess`/`onShowError`.
2. Route `export_draft` and `audit_response_copy_override` through the shared `draftTauriCommands` wrappers introduced in fix(components): inject tauri calls into useResponseActions #62.
3. Generalize `exportDraft` to accept all three formats (`Markdown`, `PlainText`, `Html`) via a new `ExportDraftFormat` union.

Why

Two of the three Wave 5 post-audit findings:

• Naming inconsistency — this hook pre-dates the `onShow*` convention from Wave 5.7 and was the only holdout.
• Purity violation — same issue fixed in fix(components): inject tauri calls into useResponseActions #62 for `useResponseActions`. This hook was the last one still calling `invoke()` directly.

How

1. Update `useResponsePanelCopy.ts` param names and substitute the two `invoke` calls with imports from `./draftTauriCommands`.
2. Extend `draftTauriCommands.ts`: `ExportDraftParams.format` widened from `"Markdown"` to a new exported `ExportDraftFormat` union.
3. Update the single consumer `ResponsePanel.tsx` to pass `onShowSuccess: showSuccess`, `onShowError: showError`.

Testing

• `pnpm tsc --noEmit` — clean.
• `pnpm test` — 241 pass (unchanged count, same coverage).
• `pnpm ui:gate:static` — exit 0.

Risk / Notes

• Runtime behavior unchanged. Same tauri commands, same payloads.
• No public API changes. `DraftTabHandle` still pinned; `ResponsePanel` props unchanged.
• All 13 Wave 5 Draft hooks now follow one contract: state + deps in via options object, `onShow*` callbacks for toast surfaces, zero direct side-effect imports.