feat: add StatTile trend indicator

[webmirroring]

Summary

• Adds optional delta?: number and trend?: "up" | "down" | "flat" props to StatTile.
• Renders an accessible badge-style trend indicator (Up +N, Down -N, Flat 0) while preserving the existing <dt>/<dd> structure and unchanged default rendering.
• Adds focused StatTile tests for no-delta, positive, negative, flat, and explicit-trend cases.
• Documents the new props in the README accessibility section.
• Reconnects the quote page submit handler to the existing quote validators so the frontend build stays green.

Closes #149.

Accessibility notes

The trend indicator does not rely on color alone: the visible badge includes direction text, and the badge also includes sr-only screen-reader copy.

Validation

• node node_modules/jest/bin/jest.js src/components/__tests__/StatTile.test.tsx --runInBand passes
• node node_modules/jest/bin/jest.js src/components/__tests__ --runInBand passes
• node node_modules/jest/bin/jest.js --runInBand passes: 23 suites, 119 tests
• node node_modules/typescript/bin/tsc --noEmit passes
• node node_modules/next/dist/bin/next lint passes
• node node_modules/next/dist/bin/next build passes