fix: keep the unsupported-flag warning off browsers

--enable-blink-features is on the list of flags Chromium warns about, so every
browser the flag was applied to put a yellow "unsupported command-line flag"
bar above each page.

Browsers are given --enable-features=MiddleClickAutoscroll instead. It asks for
the same thing - Blink generates a feature of the same name for each of its
runtime flags - and is not on that list. That spelling only works from Chromium
124 onwards, so everything else keeps the flag that works everywhere: an
application embedding an older Chromium, Steam's CEF among them, has no such
bar to show anyway.

Helium knows the feature under a name of its own and ignores the Chromium one,
so browsers are asked for HeliumMiddleClickAutoscroll as well; autoscroll never
worked there before. A name a browser does not know is ignored, which is what
makes one list safe for all of them.

An installation set up by an earlier version is taken back and written again
once, because a file that is already marked as patched would otherwise be left
alone with the old flag in it.

src/lib/config.sh

@@ -154,10 +154,18 @@ mca_config_load() {
 	CFG_SKIP="$(mca_config_get Skip '')"
 }
 
+# mca_flags [kind]
 # The full argument string that gets injected. Kept in one place so the flag
 # file writer and the desktop entry writer cannot drift apart.
+#
+# A browser gets the flag that leaves it without a warning bar; everything else
+# gets the one that works in every Chromium there is. See MCA_FLAG.
 mca_flags() {
-	printf '%s' "$MCA_FLAG"
+	if [[ ${1:-app} == browser ]]; then
+		printf '%s' "$MCA_BROWSER_FLAG"
+	else
+		printf '%s' "$MCA_FLAG"
+	fi
 	[[ -n ${CFG_EXTRA_FLAGS:-} ]] && printf ' %s' "$CFG_EXTRA_FLAGS"
 	printf '\n'
 }