test: add coverage for Toasts.tsx

[royalTreasure]

closes #659

Summary

Toasts.tsx (the component rendering toast state) had no test coverage — only the underlying store (toast.test.ts) was tested. Added Toasts.test.tsx covering the component's per-kind rendering logic.

Changes

• New apps/web/src/__tests__/components/Toasts.test.tsx, seeding the real useToastStore via setState (matching this repo's convention, e.g. VaultPanel.test.tsx's use of useWalletStore.setState).

Coverage added

• Each toast kind (success/error/info) renders its correct icon and style classes.
• An error toast gets role="alert" and aria-live="assertive""; success/info` toasts get neither.
• Clicking a toast's dismiss button calls the store's dismiss with that toast's id, leaving other toasts untouched.

Verification

• npx vitest run src/__tests__/components/Toasts.test.tsx — 4/4 pass.
• Full apps/web suite: 83/83 tests pass (including the 4 new ones). 3 pre-existing test files (unrelated to Toasts) fail to even load, independent of this change — they error on resolving the @meridian/shared workspace import (e.g. useVaultActions.test.ts), which looks like a pre-existing workspace-linking/build gap in this environment, not something this PR touches.

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