Add user theme support and migrate frontend to HTMX templates

- Add theme column to users table with default 'tokyo-night'
- Update all user queries to include theme field
- Add ListUsers and UpdateUserTheme database queries
- Update auth handlers to support HTMX form submissions and JSON API
- Add ListUsers API endpoint
- Replace embedded static files with Go templates
- Update Dockerfile to copy templates directory
- Redesign index.html with inline styles and HTMX forms
- Update Bruno API testing requests for auth endpoints

backend/migrations/001_create_tables.up.sql

@@ -4,6 +4,7 @@ CREATE TABLE users (
     email VARCHAR(255) UNIQUE NOT NULL,
     username VARCHAR(255) UNIQUE NOT NULL,
     password_hash VARCHAR(255) NOT NULL,
+    theme VARCHAR(50) DEFAULT 'tokyo-night',
     created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
     updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
 );