content: Follow web app in using Source Sans 3, size 14, height 17

From looking at the "Computed" tab in the Chrome DevTools element
inspector, on CZO at 8.0-dev-2850-gef1c1ce05f.

Technically the web app has a computed `line-height` of 16.996px,
but that seems like a rounding error.

One reason for doing this now is to opt out of a default that would
take effect with the upcoming migration to Material 3. The text
would otherwise get a line height of 1.43 times the font size, and
we want it to be denser than that. We briefly explored how to
preserve the line height exactly, across the M3 migration, but the
solutions we found seemed more awkward than just taking care of
this:
  zulip#380 (comment)

Fixes-partly: zulip#294

lib/widgets/content.dart

@@ -104,7 +104,14 @@ class Paragraph extends StatelessWidget {
     // The paragraph has vertical CSS margins, but those have no effect.
     if (node.nodes.isEmpty) return const SizedBox();
 
-    final text = _buildBlockInlineContainer(node: node, style: null);
+    final text = _buildBlockInlineContainer(
+      node: node,
+      style: const TextStyle(
+        fontFamily: 'Source Sans 3',
+        fontSize: 14,
+        height: (17 / 14),
+      ).merge(weightVariableTextStyle(context)),
+    );
 
     // If the paragraph didn't actually have a `p` element in the HTML,
     // then apply no margins.  (For example, these are seen in list items.)

test/widgets/content_test.dart

@@ -65,7 +65,7 @@ void main() {
   group('LinkNode interactions', () {
     // The Flutter test font uses square glyphs, so width equals height:
     //   https://github.com/flutter/flutter/wiki/Flutter-Test-Fonts
-    const fontSize = 48.0;
+    const fontSize = 14.0;
 
     Future<void> prepareContent(WidgetTester tester, String html) async {
       await testBinding.globalStore.add(eg.selfAccount, eg.initialSnapshot());