docs: add typography scale documentation

[Hollujay]

Summary

Creates docs/TYPOGRAPHY_SCALE.md documenting the full typography system used across the RemitFlow frontend, along with fixes for pre-existing ESLint errors.

Changes

New: docs/TYPOGRAPHY_SCALE.md

• Documents the complete type scale (xs through 4xl) with actual rem/px values pulled from the codebase CSS
• Covers line-height tokens (tight, compact, default, relaxed) and their usage
• Documents font-weight options (regular through extrabold) with real component examples
• Font family guidance: default system sans-serif stack vs monospace for amounts/addresses
• Rationale section explaining the scale's design principles
• Real CSS code examples from Home.css, Modal.css, Button.css, StatusBadge.css, QuoteCard.css, and DataTable.css
• Usage checklist for contributors
• Audience: written for contributors adding/editing components

Updated: README.md

• Added a "Documentation" section linking to the typography scale guide

Fixed: ESLint lint errors

• Added Node.js globals to test files config (fixed process is not defined in 6 test files)
• Added Vitest globals to test files config (fixed vi is not defined)
• Disabled no-unused-vars for test files (standard pattern — components imported but tested via render(<App />))
• Removed unused err variable in test/services/wallet.test.js catch block
• Removed unused MIN_TARGET constant in test/touch-targets.test.js
• Added eslint-disable comment for intentionally unused _payload parameter in src/services/wallet.js

Fixed: Test regex

• Changed /to/i to /^to$/i in test/integration/send-money-form.test.jsx to avoid matching ThemeToggle's aria-label="Switch to light mode"

Validation

• npm run lint — ✅ 0 errors, 0 warnings
• npm run build — ✅ builds successfully
• npm test — ✅ 22 test files, 128 tests, 0 failures

Linked Docs

• README.md (new Documentation section)

Closes #1174