fix(qt): handle pixel-sized fonts when scaling widgets

[thepastaclaw]

Pixel-sized font scaling fix

Issue being fixed or feature implemented

GUIUtil::updateFonts() assumed every widget font had a positive point size. The recovery-phrase dialog stylesheet uses pixel-sized fonts, for which Qt reports an invalid point size, so an asynchronous global font refresh could abort while the dialog was open. This caused the original macOS report in #7281 and the cross-platform recurrence in #7464.

Fixes #7281.
Fixes #7464.

Supersedes the earlier closed draft #7315, which GitHub would not reopen after the branch was rebased onto current develop.

What was done?

• Convert valid pixel-sized widget fonts to point sizes using the widget's logical DPI before applying the existing scaling path.
• Skip widgets when Qt provides neither a valid point size nor a safely convertible pixel size.
• Preserve fractional point sizes in the per-widget cache.
• Add a focused Qt regression test covering a pixel-sized font passed through GUIUtil::updateFonts().

How Has This Been Tested?

• Built qt/libbitcoinqt.a and test_dash_qt against current develop.
• ./src/qt/test/test_dash-qt passes with the default minimal platform. On macOS, the focused font-test body is skipped under minimal, matching the existing AppTests platform limitation.
• QT_QPA_PLATFORM=cocoa ./src/qt/test/test_dash-qt exercises and passes FontTests; the later full AppTests suite cannot run to completion in this headless macOS environment because Cocoa pasteboard initialization aborts.
• git diff --check upstream/develop...HEAD passes.
• Independent pre-PR code-review gate passes after target-branch integration validation.

Breaking Changes

None.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation, where applicable
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[thepastaclaw]

The linux64_ubsan-test / Test source failure is unrelated to this PR, so the branch is proper as-is and has been left untouched.

The failing test is feature_llmq_connections.py, which timed out waiting for DKG phase 2 / receivedContributions on all three in-job attempts. This PR changes only Qt font-scaling and Qt unit-test files; it does not change the functional test, its framework, or LLMQ code. The exact develop base passed the same UBSan test in 82 seconds, and this PR's normal linux64-test job passed as well.

Filed #7466 with the failure logs and reproduction evidence. No commit, push, rebase, or CI retrigger is appropriate for this PR.