docs: add API_REFERENCE.md — developer reference for backend API client

[CodedMumu]

Summary

Developers had no reference for how to call the backend API. This PR adds API_REFERENCE.md — a complete developer-facing reference for every function in src/lib/api.ts.

What was added

A 450-line markdown document covering:

• Environment configuration — NEXT_PUBLIC_API_URL table with required/default/purpose columns
• Response types — all exported TypeScript interfaces (Project, ProjectWithDetail, Investment, PaginatedProjectsResponse, PricePoint) with field-by-field descriptions
• 6 endpoints, each with:
  • Function signature with types and defaults
  • Backend HTTP method + path
  • Parameter table with constraints
  • Return value description
  • Demo/fixture fallback behaviour
  • Copy-paste usage example
• Error handling contract — explains the never-throw-on-network pattern and the one exception (createInvestment throws on invalid input)
• Demo/fixture fallback — explains NEXT_PUBLIC_API_URL-absent mode in detail
• End-to-end usage examples — lazy pagination, project detail page, deposit flow, sparkline chart

Endpoints documented

Function	HTTP	Description
getProjects	GET /projects	All bond projects
getProjectsPaginated	GET /projects?page=&limit=	Lazy-load paginated slice
getProject	GET /projects/:id	Single project + detail
createInvestment	POST /investments	Create investment record
getPriceHistory	GET /projects/:id/price-history	30-day price/yield history
biometricLogin	Client-only (WebAuthn)	Face ID / Touch ID auth

What was tested

No runtime code was changed — documentation only. Verified the document renders correctly in markdown preview.

close #441

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