feat: implement dynamic changelog loading from API

- Add GET /api/changelog endpoint to fetch changelog from CHANGELOG.md
- Create service/changelog.go to parse CHANGELOG.md markdown file
- Add handler/changelog.go to handle changelog requests
- Update ChangelogDrawer component to fetch from API instead of hardcoded data
- Export apiFetch from lib/api.ts for public use
- Add changelog parser tests with 14 version entries verified

AGENTS.md

@@ -2,9 +2,11 @@
 
 Guidelines for AI coding agents working on BillAI - a microservices bill analysis system.
 
+**Current Version:** 1.3.1 | Go: 1.24.0 | Node: 20+ | Python: 3.12+
+
 ## Architecture
 - `web/` - SvelteKit 5 + TailwindCSS 4 + TypeScript (Frontend, port 3000)
-- `server/` - Go 1.21 + Gin + MongoDB (API, port 8080)
+- `server/` - Go 1.24 + Gin + MongoDB (API, port 8080)
 - `analyzer/` - Python 3.12 + FastAPI (Data cleaning, port 8001)
 
 SvelteKit proxies `/api/*` requests to Go backend via `web/src/routes/api/[...path]/+server.ts`.
@@ -112,3 +114,10 @@ Upload: ZIP/XLSX → Extract → Convert UTF-8 CSV → Detect bill type → Dedu
 | `analyzer/server.py` | FastAPI entry |
 | `analyzer/cleaners/base.py` | BaseCleaner ABC |
 | `analyzer/category.py` | Category inference |
+
+## Agent Guidelines
+- **Before coding:** Search codebase to understand existing patterns and dependencies
+- **Dependencies:** Check `package.json`/`go.mod`/`requirements.txt` before adding new packages
+- **Tests:** Always run relevant test suite before committing changes
+- **Git commits:** Provide clear messages explaining the "why" of changes
+- **File references:** Use `file_path:line_number` format when mentioning code locations