fix(modal): prevent scroll chaining to the page on iOS Safari

[presidoclintonbased-alt]

Problem

Modal locks page scroll through useModalOverflow, which only sets body { overflow: hidden }. On iOS Safari that isn't enough:

1. Nothing contained overscroll. When a scroll inside the modal hit its top or bottom, Safari chained it to the page, which rubber-banded or scrolled behind the modal.
2. The dialog wasn't a scroll container. It had no max height and no overflow-y, so a tall modal on a phone (e.g. Buy Policy, Deposit) grew past the viewport and every touch-scroll went straight to the page.

Changes

Commit 1: page-level lock (useModalOverflow + globals.css)

• While any modal lock is held, the hook adds a modal-scroll-locked class (exported as SCROLL_LOCK_CLASS) to <html>, and removes it once the last lock is released or resetModalOverflow() runs. It uses the same activeLocks set, so stacked modals still work.
• globals.css: html.modal-scroll-locked, html.modal-scroll-locked body { overscroll-behavior: none; } stops the page itself from rubber-banding or chaining.
• A class is used instead of inline styles so the rule lives in CSS and doesn't depend on reading back and restoring previous inline values.

Commit 2: dialog-level containment (Modal.tsx)

• The dialog is now its own scroll area: max-h-[calc(100dvh-2rem)] overflow-y-auto overscroll-contain. dvh tracks iOS's collapsing toolbar, and 2rem matches the backdrop's p-4.
• The backdrop also gets overscroll-contain.
• I didn't use the issue's alternative, touch-action: none on the overlay. Browsers apply it to descendants as well, so it would also stop users scrolling inside the modal.

Tests

• useModalOverflow.test.ts: the class is present only while at least one lock is held (checked with two stacked locks), and resetModalOverflow() clears it.
• Modal.test.tsx: the dialog and backdrop have the containment and viewport-cap classes.

Verification

• npx vitest run src/__tests__/useModalOverflow.test.ts src/__tests__/Modal.test.tsx: all new tests pass (6/6 hook, and the new Modal test).
• Pre-existing on main, not caused by this PR: Modal.test.tsx › focuses a focusable element on open already fails, and tsc already reports a type error in BuyPolicyModal.test.tsx. No new type errors.
• Not yet checked on a physical iOS device. A quick check on iOS Safari (open a modal, scroll to its top and bottom) is recommended before merge.
  Closes [bug] Modal does not prevent scroll chaining on iOS Safari #552
  Closes [security] Wallet address displayed in page title via metadata #553
  Closes [ux] EmptyState component does not support custom action button styling #554
  Closes [bug] Claims export downloads CSV with wrong filename timestamp #555

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