fix: render the message pane at all

Clicking a thread left the pane blank. Two independent bugs, both from the
same false premise: that setHtml() navigates to the base URL it is given.

It does not. setHtml() navigates to a data: URL carrying the markup and
applies the base URL afterwards, purely as the document's origin. Verified
empirically on Qt 6.11.

Built on that wrong assumption were:

- MessagePage::acceptNavigationRequest compared the navigation's URL
  against documentUrl() and rejected everything else, so the document load
  was refused. It now accepts a typed main-frame navigation, which is one
  we initiated ourselves.
- RequestInterceptor exempted exactly the qtmaildir: base URL and denied
  everything else, so the data: document load was blocked too.

The interceptor fix is scoped to ResourceTypeMainFrame rather than allowing
the data: scheme outright. A blanket allow would have been a real hole: a
message body can write <img src="data:..."> or an iframe, and the existing
dataSchemeBlocked test in test_interceptor.cpp was right to fail when that
was tried. Sub-resource data: URLs remain denied.

Note this was never working. The drafted version had the same defect in a
different spelling (it compared url.scheme() rather than the whole URL, and
would have rejected the data: navigation just the same), and task 11 shipped
with no runtime test to catch it. test_messageview.cpp now pins all three
facts: the document loads, its text reaches the page, and a data: image
inside a hostile body stays blocked.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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