test(components): cover useResponsePanelCopy — 23 tests, 0 prior

The 160-line useResponsePanelCopy hook exposes 8 return-surface members
(copied, showCopyOverride, copyOverrideReason, copyOverrideSubmitting,
showExportMenu, exportMenuRef, plus 4 handlers and 3 setters) and
invokes the Tauri command boundary (auditResponseCopyOverride,
exportDraft) plus navigator.clipboard — yet had zero tests. This was
the most-cited AGENTS.md DoD gap in the audit.

Adds useResponsePanelCopy.test.ts with 23 tests grouped by handler:

handleExport (4)
  - happy path: exportDraft payload + success toast + menu closes
  - empty response short-circuits before calling the command
  - exception path: onShowError fires, menu still closes
  - saved=false: no success toast, no error, command was called

handleCopy (8)
  - happy path: writes navigator.clipboard with the raw response
  - uses parsed.output when hasSections is true
  - copied flag resets to false after the 2s timeout (fake timers)
  - opens override modal when confidence mode is not 'answer'
  - opens override modal when there are zero citations
  - empty response short-circuits
  - clipboard rejection routes to onShowError without flipping copied
  - undefined confidenceMode defaults to 'answer' for gating

handleConfirmCopyOverride (7)
  - happy path: audits, writes clipboard, success toast, modal closes,
    reason cleared
  - undefined confidenceMode is sent to audit as null
  - empty reason: onShowError, no audit, no clipboard
  - whitespace-only reason: same as empty (trim applied)
  - audit failure: error toast, submitting flag resets in finally
  - empty response short-circuits
  - uses parsed.output when hasSections is true

cancelCopyOverride (1)
  - closes modal and clears reason atomically

Export-menu click-outside useEffect (3)
  - outside mousedown closes the menu via the ref contract
  - inside mousedown leaves the menu open
  - no mousedown listener is registered while the menu is closed

Mocks only external boundaries: the Tauri command module is mocked via
vi.mock("./draftTauriCommands", ...) with hoisted mock fns;
navigator.clipboard.writeText is installed on the jsdom navigator.

Total: 69 test files / 264 tests (was 68 / 241 on master). Typecheck
and eslint clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: saagar210