feat(web): pause toast auto-dismiss on hover and focus

[DevQwinB]

closes #653

Summary

push() in apps/web/src/store/toast.ts fired an unconditional 4-second
setTimeout for every toast, and nothing in Toasts.tsx paused it. An error
toast with a long message got exactly the same window as a one-word success
toast, and it disappeared mid-read no matter what the user was doing.

Changes

apps/web/src/store/toast.ts

• TOAST_DURATION_MS (exported) per kind: error 8000ms, success/info
  4000ms — unchanged for the two short kinds.
• Dismiss timers moved into a module-level map keyed by toast id: they're
  imperative handles, not rendered state, so pausing/resuming never re-renders
  the toast list.
• New pauseTimer(id) / resumeTimer(id) actions. Pause banks the remaining
  time; resume schedules exactly that remainder, so a toast hovered several
  times still disappears after its full duration of un-hovered time rather
  than restarting each cycle. A repeated pause without an intervening resume
  (moving between the message and the dismiss button fires two mouseenters)
  is a no-op, so elapsed time is never subtracted twice.
• dismiss() now also cancels the pending timer instead of leaving it to fire
  against an already-removed id.
• toasts, push, and dismiss keep their existing signatures.

apps/web/src/components/ui/Toasts.tsx

• onMouseEnter/onFocus pause, onMouseLeave/onBlur resume. Focus events
  bubble in React, so tabbing to the dismiss button pauses the toast too — a
  keyboard user is reading it just as much as a hovering one.
• The close button still dismisses immediately, including while hovered.

Verification

apps/web:

• pnpm test → 18 files, 120 tests passing (13 store + 4 new Toasts.test.tsx
  component tests, covering: hover pauses and mouse-leave resumes, focus/blur
  pauses and resumes, close button still works while hovered, error window
  outlives the success window, repeated pauses don't lose time, several
  hover/unhover cycles, unknown-id no-ops, dismiss cancels the timer).
• pnpm coverage → thresholds pass (85.65% stmts / 71.32% branches).
• pnpm typecheck, pnpm lint, prettier --check → clean.

The pre-existing auto-dismisses after 4 seconds test is untouched and still
passes.

Note: a fresh checkout needs pnpm build in packages/shared and
packages/stellar-sdk-helpers first, otherwise apps/web's hook tests can't
resolve @meridian/stellar-sdk-helpers. Pre-existing and unrelated to this change.

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