Feature: Billable hours tracking and invoice generation

[gloomcheng]

Problem

As a freelance developer, I need to track billable vs non-billable hours and generate invoices for clients.

Current State

Toki tracks time per project but has no concept of:

• Billable vs non-billable
• Hourly rates
• Client invoicing

Proposed Solution

1. Billable flag - Mark projects/activities as billable
2. Rate configuration - Set hourly rates per project or client
3. Invoice generation - Export billable hours as invoice (PDF/CSV)
4. Client grouping - Group projects by client for billing

Example

```bash
toki config set project.myapp.billable true
toki config set project.myapp.rate 150

toki invoice generate --client "Acme Corp" --period "2024-12"

Generates invoice PDF with line items

```

Related Pain Points

• Freelancers need accurate billing
• Current tools don't connect tracking to invoicing