Fix Linux startup crash caused by the update notification dialog

[tadeasf]

Hi! I love pyfa - I've been using it for years. I'm primarily on Linux these days, and it would crash on startup, so I decided to fix that and give some love back to the project. More to follow if you're happy with my work (I was thinking about the dark mode request, but then again, on Linux it's already sort of there, so I'd need to switch machines... eh, we'll see).

What / why

Addresses #2740 - the Linux AppImage dies ~2-3 s after launch with nothing in the Python log.

Root cause

The update notification dialog (gui/updateDialog.py) is the only user of wx.html2.WebView. On wxGTK that's WebKitGTK, and:

1. AppImage: the builder bundles jammy's libwebkit2gtk-4.0-37. The library loads, but WebKit then tries to spawn its helper processes at the Ubuntu-hardcoded path /usr/lib/x86_64-linux-gnu/webkit2gtk-4.0/WebKitNetworkProcess, which doesn't exist on other distros - a fatal GLib error that aborts the app with no Python traceback. This is why it "appeared" with 2.66.2: git diff v2.66.1..v2.66.3 touched nothing in the startup/update path - 2.66.2 was simply the first release new enough for the popup to run at all. Reproduced on Fedora 44 with the v2.64.2 AppImage (exit 133 = SIGTRAP):

   ** (pyfa.py:856537): ERROR **: 20:41:04.016: Unable to spawn a new child process:
   Failed to spawn child process "/usr/lib/x86_64-linux-gnu/webkit2gtk-4.0/WebKitNetworkProcess"
   (No such file or directory)
   

   In ~/.pyfa/pyfa.log, every crashed launch ends exactly at service.update: Starting Check Update Thread.

2. From source on distros without webkit2gtk-4.0 (Fedora today): import wx.html2 itself fails, so startup dies before the main window - gui/mainFrame.py imports the dialog at module level.

Changes

• gui/updateDialog.py: never use the WebView on wxGTK (a native abort can't be caught with try/except) - release notes render in a read-only wx.TextCtrl instead; the Download button already opens the release page in a browser. Other platforms keep the WebView but check IsBackendAvailable() and guard WebView.New()/SetPage(), falling back to plain text on failure. import wx.html2 is now lazy so startup doesn't depend on WebKitGTK being present. Two small guards while there: releaseInfo['body'] may be None (would TypeError), and the dialog title contained a literal {} - it now shows the release tag.
• gui/mainFrame.py: ShowUpdateBox wrapped in try/except + logging so a dialog failure can never silently break startup again.
• dist_assets/linux/AppImageBuilder.yml: drop libwebkit2gtk-4.0-37 - nothing loads it on Linux anymore. Happy to split this into a separate PR if preferred.

Behavior on Windows/macOS is unchanged (WebView2/WKWebView still used, now guarded).

Verified

• Reproduced pre-fix: AppImage crash (signature above), and from-source startup failure at import wx.html2, both on Fedora 44.
• Post-fix: app starts and stays up from source (venv, wxPython 4.2.1 gtk3 wheels); faked an old version in version.yml to force the popup - app survives, dialog opens with plain-text notes; headless UpdateDialog construction against live release data passes; flake8 (tox pep8 flags) adds zero findings on changed files.
• Not verified: a rebuilt AppImage (needs appimage-builder on Ubuntu) - but the WebKitGTK code path is gone entirely on Linux.

Notes

Flagging so that you know: service/update.py swallows the primary endpoint's error silently (lines 54-59) and assumes the response is a JSON list (line 61) - might be worth fixing later?