Add link to project proposal (PDF) on the project details page

[EdiWeeks]

Enhancement

Add a link to the project's proposal PDF on the project details page so it's accessible directly from Thyme — no jumping back to BC or hunting through SharePoint to find it.

Where

Project details page header, alongside the existing actions:

[Costs hidden] [Export PDF ▾] [Open in Business Central] → add: [📄 Proposal]

(or wherever fits naturally — a small link/icon under the project name would also work).

Open question — where does the proposal live?

This needs deciding before implementation. Options:

1. BC Attachments on the project — BC's Project Card already has an Attachments tab (Summary → Attachments). If proposals are uploaded there, surface the most recent one (or the first matching by name pattern). Needs the Thyme BC extension to expose attachments via the /projects endpoint (or a sibling endpoint).
2. External URL (SharePoint, Drive, etc.) — store a direct link as a field on the project. Cleanest if the team already keeps proposals outside BC. Needs a new field (similar to Display PO number (External Document No.) on project details header #205 for PO, Add project description field via Thyme BC extension and display on project details page #206 for description).
3. Both — single field that takes either a URL or an attachment reference; pick whichever is set.

The right path depends on where the team currently keeps proposals. Worth confirming with @EdiWeeks / @BenGWeeks before scoping.

Acceptance criteria

• When a proposal exists for a project, a clear link / button appears on the project details page that opens the PDF (new tab).
• Hidden when no proposal is set.
• Link is also included in the Thyme PDF export so customer-facing exports reference the proposal source.
• If using external URLs, validate the URL points to a downloadable PDF (or at least a sensible file) before showing the link.

Notes

• If we go with option 1 (BC Attachments), this is a cross-repo change like Add project description field via Thyme BC extension and display on project details page #206 — needs a companion issue in knowall-ai/thyme-bc-extension to expose attachments on the API.
• If option 2 (URL field), it's one new field on the BC extension's /projects endpoint.