WebView-v2026.04.0

WebView v2026.04.0

First CalVer-tagged WebView release. Cut from the webview-fix PR
branch (PR #2767) ahead of merge so the viewer image build can fetch
the published artifacts on first try.

Highlights vs. the previous WebView-v0.3.12 release:

* Pi 4 64-bit (pi4-64) now produces a Qt 6 build alongside pi5 and
  x86, instead of being lumped with the legacy Qt 5 cross-compile.
* WebView binary, D-Bus service / object path, install dirs, and
  on-disk resources renamed from Screenly* to Anthias* (the parent
  Screenly Inc. attribution and ~/.screenly/ legacy paths are
  unchanged).
* Several latent correctness bugs fixed: stale image-reply race,
  animated GIF buffer ownership, page-load signal handler races,
  authenticationRequired signal-slot signature mismatch.
* Server host/port are now LISTEN/PORT env-overridable instead of
  hardcoded to anthias-server:8080.
* Artifact filenames switch to webview-<calver>-<debian>-<board>.tar.gz
  (Qt version and git short hash dropped); the Qt 5 toolchain
  archive (qt5-5.15.14-bookworm-<board>.tar.gz) keeps its Qt version.