docs: Add a PLANKA Pro section to the README

Pro funds the Community edition, but the README mentioned it only in a single
line under How to Deploy. The new section sits between the demo and the key
features: what Pro adds, two buttons, four screenshots from 2.5.0, a
Community-versus-Pro table, and what an upgrade involves.

Every row of that table was checked against both codebases rather than taken
from the comparison matrix on the website, which carries cells that are
demonstrably wrong. The dangerous error is denying Community something it has,
because the reader has the opposite in front of them in their own instance —
so the stopwatch, webhooks, the REST API, custom fields, two-factor and the
notification providers are all marked as shared, and only what the code
actually gates is marked Pro. Where a claim could not be backed by the source
it was left out.

The upgrade paragraph names the releases the promote accepts as a snapshot
rather than as a standing fact, and points at the guide for the current list.
A hard-coded version window in the README of the repository whose version
keeps moving is wrong the moment the next minor ships, and nobody would
remember it was there.

The screenshots are referenced through raw.githubusercontent.com, so they
appear once this is pushed, the same way the logo and the demo already do.

README.md

@@ -8,12 +8,63 @@
 
   ![Version](https://img.shields.io/github/package-json/v/plankanban/planka?style=flat-square) [![Docker Pulls](https://img.shields.io/badge/docker_pulls-8M%2B-%23066da5?style=flat-square&color=red)](https://github.com/plankanban/planka/pkgs/container/planka) [![Contributors](https://img.shields.io/github/contributors/plankanban/planka?style=flat-square&color=blue)](https://github.com/plankanban/planka/graphs/contributors) [![Chat](https://img.shields.io/discord/1041440072953765979?style=flat-square&logo=discord&logoColor=white)](https://discord.gg/WqqYNd7Jvt)
 
-  [Install](https://docs.planka.cloud/docs/installation/docker/production-version/) ·  [Demo](https://planka.app) · [Docs](https://docs.planka.cloud/docs/welcome/) · [API](https://plankanban.github.io/planka/swagger-ui/) · [Cloud](https://planka.app/pricing) · [Pro version](https://planka.app/pro)
+  [Install](https://docs.planka.cloud/docs/installation/docker/production-version/) ·  [Demo](https://planka.app) · [Docs](https://docs.planka.cloud/docs/welcome/) · [API](https://plankanban.github.io/planka/swagger-ui/) · [Cloud](https://planka.app/pricing) · [Pro version](https://planka.app/pro?ref=github-readme)
 
   ![Demo](https://raw.githubusercontent.com/plankanban/planka/master/assets/demo.gif)
 
 </div>
 
+---
+
+<div align="center">
+
+  ## PLANKA Pro
+
+  **When the whole team runs on PLANKA.**
+
+  Phones and tablets · five themes · a dashboard across every project · calendar, timeline and
+  map views · cards on more than one board · templates · guest roles · SSO · PDF export.
+  Self-hosted exactly as you host it now, supported by the people who build it.
+
+  [![Try Pro free for 30 days](https://img.shields.io/badge/Try_Pro_free_for_30_days-22252a?style=for-the-badge)](https://planka.app/trial?ref=github-readme-trial)
+  [![Features and pricing](https://img.shields.io/badge/Features_and_pricing-5a6c78?style=for-the-badge)](https://planka.app/pro?ref=github-readme-pro)
+
+  <img src="https://raw.githubusercontent.com/plankanban/planka/master/assets/pro/dashboard.png" width="820" alt="Personal dashboard across every project">
+
+  <img src="https://raw.githubusercontent.com/plankanban/planka/master/assets/pro/mobile-board-small.png" height="240" alt="A board on a phone">
+  <img src="https://raw.githubusercontent.com/plankanban/planka/master/assets/pro/theme-picker-crop.png" height="240" alt="Theme picker">
+  <img src="https://raw.githubusercontent.com/plankanban/planka/master/assets/pro/timeline.png" height="240" alt="Timeline view">
+
+</div>
+
+| | Community | Pro |
+| --- | :---: | :---: |
+| Boards, lists, cards, real-time collaboration | ✓ | ✓ |
+| Markdown, attachments, comments, custom fields | ✓ | ✓ |
+| Stopwatch and due dates on cards | ✓ | ✓ |
+| Notifications through 100+ providers | ✓ | ✓ |
+| Webhooks and a documented REST API | ✓ | ✓ |
+| Two-factor authentication | ✓ | ✓ |
+| Self-hosted, your database, your backups | ✓ | ✓ |
+| **Free to self-host** | **✓** | — |
+| Phones and tablets, five themes, accent color | — | ✓ |
+| Calendar, timeline, map and media views | — | ✓ |
+| Recurring cards, card and board templates | — | ✓ |
+| One card on more than one board | — | ✓ |
+| Guest roles, SSO (OIDC) | — | ✓ |
+| PDF and HTML export | — | ✓ |
+| Support from the developers, no ticket queue | — | ✓ |
+
+Pro upgrades an existing Community installation in place — your database, your volume, your
+data. It is a real migration rather than a new image: Pro adds a Valkey service and a one-off
+schema promote. The promote accepts only certain Community releases as a starting point —
+currently 2.1.x and 2.2.x — and from any other version it stops without changing anything.
+**The upgrade is one-way — there is no downgrade back to Community — so take a backup first,
+or try it on a copy.** The step-by-step guide is in the customer center once you have a key,
+and lists the releases the promote accepts.
+
+---
+
 ## Key Features
 
 - **Collaborative Kanban Boards:** Create projects, boards, lists, cards, and manage tasks with an intuitive drag-and-drop interface
@@ -28,7 +79,7 @@ PLANKA is easy to install using multiple methods - learn more in the [installati
 
 For configuration and environment settings, see the [configuration section](https://docs.planka.cloud/docs/category/configuration/).
 
-Interested in a hosted or [Pro version](https://planka.app/pro) of PLANKA? Check out the pricing on our [website](https://planka.app/pricing).
+For hosted plans and pricing, see our [website](https://planka.app/pricing?ref=github-readme).
 
 ## Notes App