feat(frontend): add Storybook for UI components

[Macnelson9]

Summary

• Wires @storybook/html-vite to frontend/ with dark/light theme switcher via preview.ts
• Adds 7 component stories under frontend/src/stories/:
  • HfBadge — safe / warning / critical / infinite states
  • LeverageSlider — default, high-leverage, expert mode, no-zones
  • PoolCard — Etherfuse, Fixed, frozen pool variants
  • AssetPicker — interactive tab bar with optional balance display
  • AprCard — supply / borrow / with-extras variants
  • Toast — success / info / error / stack variants
  • StatCard — health factor, total supply, loading skeleton, negative trend
• Adds storybook and build-storybook scripts + devDependencies to frontend/package.json

Test plan

• cd frontend && npm install && npm run storybook — Storybook opens at http://localhost:6006 with all 7 components visible across dark/light themes

Closes #76

🤖 Generated with Claude Code

[Dgetsylver]

Blocker: frontend/package.json gains 5 Storybook devDeps but frontend/package-lock.json isn't updated — npm ci breaks for everyone. Commit the lockfile update.

Bigger question before this merges: the stories are hand-rolled HTML mockups with inline styles and CSS variables that don't exist in frontend/src/style.css (--color-surface vs the real --surface), so they document look-alikes rather than the actual UI and will drift immediately. Suggest either importing/reusing the real markup and stylesheet, or scoping this down to the components that can be rendered faithfully.