Load review UI from file instead of inline HTML

[pokoli]

This changes Review Loop to load its web UI from the built
web/dist/index.html file instead of passing the full HTML inline to
open().

Why:

• On Linux, when glimpseui falls back to its Chromium backend, opening the
  large self-contained Review Loop HTML inline can result in a blank window.
• Loading the built file from disk avoids that problem and worked correctly in
  local testing.

What changed:

• Added a helper to resolve the built review HTML file path.
• Open the window first, then load the UI with window.loadFile(...) on
  ready.

This is a small change that should make the extension more robust on Linux
setups using the Chromium fallback backend.

[zegging]

Just hit what looks like the same underlying issue on Windows — the inline-HTML path also breaks the WebView2 backend, not just the Linux/Chromium fallback.

WebView2's CoreWebView2.NavigateToString has a hard ~2 MB limit on the HTML string (documented on ICoreWebView2::NavigateToString), and the self-contained Review Loop UI (bundles Monaco) blows past it. Result on Windows is an immediate ArgumentException from the native Glimpse host:

System.ArgumentException: Value does not fall within the expected range.
   at Microsoft.Web.WebView2.Core.Raw.ICoreWebView2.NavigateToString(String htmlContent)
   at Microsoft.Web.WebView2.Core.CoreWebView2.NavigateToString(String htmlContent)
   at GlimpseHost.HandleCommand(JsonElement json) in .../glimpseui/native/windows/Program.cs:line 233
   at GlimpseHost.HandleCommand(String payload) in .../glimpseui/native/windows/Program.cs:line 220
   at GlimpseHost.<>c__DisplayClass31_0.<ReadCommandsAsync>b__1() in .../glimpseui/native/windows/Program.cs:line 184
   at System.Windows.Forms.Control.InvokeMarshaledCallbackHelper(Object obj)
   at System.Threading.ExecutionContext.RunInternal(...)
--- End of stack trace from previous location ---
   at System.Threading.ExecutionContext.RunInternal(...)
   at System.Windows.Forms.Control.InvokeMarshaledCallbacks()

Environment: Windows, .NET SDK 8.0.423, WebView2 Microsoft.Web.WebView2.Core 1.0.2957.106, glimpseui 0.8.1, pi-review-loop 0.3.0.

Since this PR switches to loadFile(...) on the built web/dist/index.html, it should fix the Windows crash as a side effect (the offending NavigateToString call in the Glimpse Windows host stops being reached at all). Would be great to get it merged — happy to test the merged version on Windows if useful.

[zegging]

Tested this branch on Windows against the same setup from my previous comment (Win + .NET SDK 8.0.423 + WebView2 1.0.2957.106 + glimpseui 0.8.1). /diff-review now opens the Review Loop UI cleanly — no more NavigateToString ArgumentException, Monaco diff renders and the file list interactions work. LGTM from a Windows-smoke-test perspective.