Windows: Terminal panel uses \\?\ extended-length path prefix, breaking cmd.exe child processes (npm, npx, etc.)

## Description

The terminal panel in Berd spawns PowerShell with the session's `working_dir` set to the repository path. When the path is under `C:\Users\...` (or any long path), PowerShell resolves it to the Win32 extended-length path format `\\?\C:\Users\...`. This breaks any child process that spawns `cmd.exe` — including `npm`, `npx`, and many Node.js toolchains.

## Symptoms

1. **Terminal prompt shows the extended-length path format:**
```
PS Microsoft.PowerShell.Core\FileSystem::\\?\C:\Users\0\.buzz\REPOS\inferencesaver>
```

2. **Running `npm run dev` (or any npm/npx command) fails with:**
```
'\\?\C:\Users\0\.buzz\REPOS\inferencesaver'
CMD.EXE was started with the above path as the current directory.
UNC paths are not supported. Defaulting to Windows directory.
```

3. **`next dev` crashes with:**
```
Error: EISDIR: illegal operation on a directory, lstat 'C:'
```

4. **`npm run dev:docs` fails with:**
```
Could not read package.json: Error: ENOENT: no such file or directory, open 'C:\Windows\package.json'
```

## Root Cause

- Berd's terminal panel spawns PowerShell with the session's `working_dir` as the initial CWD
- When the working directory is a normal Windows path like `C:\Users\0\.buzz\REPOS\inferencesaver`, PowerShell's FileSystem provider resolves it to the extended-length path format `\\?\C:\Users\...`
- `cmd.exe` (spawned by npm/npx) does not support `\\?` UNC paths as a CWD and falls back to `C:\Windows`
- This causes all downstream Node.js processes to fail because they're running from the wrong directory
- Even Git Bash (`C:\Program Files\Git\bin\bash.exe --login`) spawned from this terminal inherits the `\\?` CWD environment

## Workaround

Run `cd C:\Users\0\.buzz\REPOS\inferencesaver` once in the terminal to reset the PowerShell provider path to the normal format. This is not obvious and wastes time every time a new terminal is opened.

## Affected Environment

- **Berd version:** v0.6.2
- **OS:** Windows 10 (build 26100)
- **Shell:** PowerShell 5.1 (default Windows PowerShell)
- **Node.js:** v24.18.1

## Suggested Fix

When spawning the terminal, Berd should normalize the `working_dir` path to its non-extended-length form (`C:\...` instead of `\\?\C:\...`) before passing it to PowerShell. This could be done by:

1. Passing the path without the `\\?` prefix when spawning the shell process
2. Or, if using PowerShell, prepending a `cd` to the non-extended path in the initial command
3. Or, using Git Bash or Windows Terminal as an alternative shell option

## Impact

This is a significant UX issue for any Windows user of Berd who works with Node.js projects, Python projects, or any toolchain that spawns `cmd.exe` subprocesses. It makes the terminal unusable out of the box for running dev servers, builds, and package managers.

Start at the terminal panel code that spawns PowerShell and passes the session's working_dir; reproduce the issue on Windows using a repository under C:\Users\... and npm or npx. The fix is complete when a new terminal starts with a normal C:\... working directory and child cmd.exe-based tools run from the repository rather than C:\Windows.

Written by the indexing model from the issue text.