docs: rewrite the README around the desktop app and simplify the badges (#103)

## Related Issue

No issue. The problem is described below.

## Problem

The README opened with thirteen `for-the-badge` badges and then spent
most of its length on
repository structure. The desktop app, which is the easiest way for a
new user to try Pythinker,
was not mentioned anywhere outside a single package table row.

## What changed

- Lead with the desktop app: download link, what it does, where its data
lives, and platform notes.
- Expand the agent capabilities into readable sections (codebase work,
subagents, tool approvals,
  MCP and skills and plugins, model choice, video input).
- Cut the badge row from 13 `for-the-badge` badges to 7 flat-square
ones: release, desktop
  downloads, license, macOS, Windows, Node.js, visitors.
- Use the desktop screenshot as the hero image
(`docs/media/desktop-ui.webp`, 24 KB, resized with
ffmpeg and encoded with cwebp). The terminal recording moves down to the
terminal section.
- Point every download link and both platform badges at
https://code.pythinker.com/.
- Drop the duplicated project-layout tree and the extensibility table,
and trim the closing blocks.

Verified: the shields and komarev badge URLs return SVG with the
expected labels and colors,
`pnpm dev:desktop` exists, the docs routes resolve, and the file has no
em or en dashes.

## Checklist

- [x] I have read the
[CONTRIBUTING](https://github.com/PyModel/pythinker-code/blob/main/CONTRIBUTING.md)
document.
- [x] I have linked a related issue, or explained the problem above.
- [ ] I have added tests that prove my feature works. (Documentation
only.)
- [x] Ran `gen-changesets` skill, or this PR needs no changeset. (No
release artifact changes.)
- [x] Ran `gen-docs` skill, or this PR needs no doc update.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
  * Reorganized the README around the desktop app experience.
* Added download guidance, getting-started instructions, interaction
guides, and platform limitations.
* Documented local runtime behavior, shared CLI data, authentication,
terminal installation, and editor integration.
* Updated development guidance for desktop, CLI, web, engine, provider,
execution, server, and SDK packages.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: elkaix