Feature Request: Project Estimates / Quotes (with conversion to Invoices)

[felipep]

Is your feature request related to a problem? Please describe.

Currently, solidtime offers great time tracking and invoice generation. However, before a project even begins, freelancers and agencies usually need to send an estimate or quote (Angebot) to the client. Right now, users have to use external tools to create these estimates, which breaks the all-in-one workflow.

Describe the solution you'd like

It would be incredibly helpful to have a "Project Estimates" (or "Quotes") feature.
The functionality could be very similar to the existing invoice creation workflow:

1. Estimate Builder: Create a document with line items, prices, and client details, just like creating an invoice.
2. Convert to Invoice: Once the client accepts the estimate and the work is done, there should be a 1-click option to "Convert Estimate to Invoice".

Describe alternatives you've considered

Currently, the only alternative is to create estimates in external tools (like Word, Google Docs, or other accounting software) and later manually recreate them as invoices in solidtime. This is time-consuming and error-prone.

Additional context

This workflow (Estimates -> Invoices) is a core feature in popular tools like Harvest. Adding this to solidtime would be a massive selling point and would easily convince many users who are currently looking for a solid, open-source alternative to Harvest to make the switch.