Buzz Term hangs the entire app on Windows (0.5.5) - banner flashes, then Application Hang; blank panel after relaunch

## Environment

- Buzz Desktop **0.5.5.0** (arrived via auto-update from 0.5.4 today)
- Windows 11 Pro, build 26200
- Self-hosted relay (`deploy/compose` bundle), Let's Encrypt TLS

## What happened

Opened Buzz Term from the channel workspace for the first time:

1. The Term banner/wordmark flashed briefly, with what looked like a local shell prompt.
2. The entire app immediately stopped responding (not just the Term panel).
3. Windows Event Log recorded an **Application Hang** for `buzz-desktop.exe` 0.5.5.0 at that moment: *"The program buzz-desktop.exe version 0.5.5.0 stopped interacting with Windows and was closed."*
4. After relaunching, the Term panel renders permanently blank. The rest of the app works normally.

## Expected

Either a working shell on Windows (ConPTY + PowerShell/cmd), or a graceful "not supported on this platform yet" state. A failed shell spawn should never hang the UI thread or take down the whole app.

## Possibly relevant

PR #4347's shell resolution chain (`$SHELL` env var → passwd database → `/bin/sh`) appears to have no valid candidate on a stock Windows install — none of those exist here. The Application Hang (rather than a crash/panic) suggests the failure path blocks the main thread while the frontend waits on a PTY that never materializes.

Happy to provide more diagnostics — though note the desktop currently writes no log files on Windows, so Event Log was the only artifact available.

Start by reviewing PR #4347's shell resolution chain and the Windows desktop Term/PTY launch path; reproduce the first-launch case on stock Windows 11. Done means Term either opens a working ConPTY shell or shows a graceful unsupported/error state, without hanging the UI or remaining blank after relaunch.

Written by the indexing model from the issue text.