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/internal/database/models.go

@@ -36,6 +36,7 @@ type Users struct {
 	Email        string             `db:"email" json:"email"`
 	Username     string             `db:"username" json:"username"`
 	PasswordHash string             `db:"password_hash" json:"password_hash"`
+	Theme        pgtype.Text        `db:"theme" json:"theme"`
 	CreatedAt    pgtype.Timestamptz `db:"created_at" json:"created_at"`
 	UpdatedAt    pgtype.Timestamptz `db:"updated_at" json:"updated_at"`
 }