feat: add sticky action bar on LinkedAccounts page (#688)

[GreatShinro]

Summary

Adds a sticky bottom action bar to the LinkedAccounts page that appears when the user scrolls past the header, providing quick access to primary wallet actions.

Closes #688

Changes

• New page: src/pages/LinkedAccounts.tsx — Full page with connected wallet info card, available wallets grid, and sticky bottom action bar
• Route: app/(dashboard)/linked-accounts/page.tsx — Next.js route wrapper
• Tests: src/pages/__tests__/LinkedAccounts.test.tsx — 20 passing tests
• Test setup: jest.setup.js — Added navigator.clipboard mock

Sticky Action Bar Behavior

• Hidden by default, appears on scroll past the page header
• Uses getBoundingClientRect for scroll detection (no scroll library dependency)
• Shows wallet name, copy address, disconnect, and view on explorer buttons
• When no wallet connected, shows a connect button instead
• Respects prefers-reduced-motion via Tailwind motion-reduce classes

Accessibility (WCAG 2.1 AA)

• role="toolbar" with aria-label="Wallet actions" on the sticky bar
• aria-live="polite" region announces wallet connection status changes
• All interactive elements have visible focus-visible rings
• Reduced motion support via Tailwind utility classes

Responsive

• Mobile (<640px): compact labels, icon-only on small screens
• Tablet+ (640px+): full action labels visible
• Desktop (1024px+): content constrained to max-w-4xl

Test Coverage (20/20 passing)

• Rendering: heading, wallet info, balance, copy/disconnect buttons, not-connected state
• Available wallets: filtering connected wallet, connect actions
• Sticky bar: toolbar role, scroll behavior, wallet info in bar, connect button
• Accessibility: aria-live region, announcements, accessible labels

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