[Windows] cookie-import-browser does not support Windows — please add local browser cookie import

## Feature Request

First, I want to say thank you for building gstack — it's genuinely changed how I work with AI on real projects. The browse skill in particular is impressive.

## The Problem

On Windows, `/setup-browser-cookies` fails with:

```
No Chromium browsers found. Supported: Comet, Chrome, Chromium, Arc, Brave, Edge
```

Even though Chrome is installed at `C:\Program Files\Google\Chrome\Application\chrome.exe`.

Looking at the source, `cookie-import-browser.ts` defines:

```ts
type BrowserPlatform = 'darwin' | 'linux';
```

And `getBaseDir()` only handles macOS (`Library/Application Support`) and Linux (`.config`). There is no Windows path (`%LOCALAPPDATA%\Google\Chrome\User Data`).

## What I Had to Do Instead

I manually exported cookies using a Chrome extension (Cookie-Editor), then discovered the exported format was also incompatible — `sameSite` values like `no_restriction` and `null` are rejected by Playwright which expects `Strict|Lax|None`. I had to write a conversion script myself before `cookie-import` would accept the file.

## What Would Be Amazing

Support Windows in `cookie-import-browser` by:

1. Adding `'win32'` to `BrowserPlatform`
2. Mapping Chrome's Windows user data dir: `%LOCALAPPDATA%\Google\Chrome\User Data`
3. Handling Windows cookie decryption (DPAPI-based, different from macOS Keychain)

I understand the DPAPI decryption part is non-trivial — even just detecting the browser and opening the picker UI on Windows would already be a huge step forward.

I'm happy to test any WIP builds on Windows 11. This is a real gap for Windows users who want to use gstack's browse skill with authenticated sessions.

## Environment

- **OS:** Windows 11 Pro 10.0.26200
- **Shell:** Git Bash (bash)
- **gstack version:** v0.15.1.0
- **Chrome:** installed at `C:\Program Files\Google\Chrome\Application\chrome.exe`
- **Affected skill:** `/setup-browser-cookies`
- **Affected file:** `browse/src/cookie-import-browser.ts`