feat: check every subscription in the background and add the posts page

app/lib/application/pages/page_blocks.dart

@@ -11,9 +11,14 @@ import 'mews_urls.dart';
 /// constant here rather than a message.
 const actionSeparator = '  ';
 
-/// One choice in a group of mutually exclusive values.
+/// A link with its label: a choice in a group, or an action on a row. For an
+/// action [current] is always false.
 typedef Choice = ({String href, String label, bool current});
 
+/// An action link on a row.
+Choice action(String href, String label) =>
+    (href: href, label: label, current: false);
+
 /// One entry in the strip of internal pages.
 typedef NavItem = ({String url, String labelId});
 
@@ -21,6 +26,7 @@ typedef NavItem = ({String url, String labelId});
 /// never shifts under the reader.
 const navItems = <NavItem>[
   (url: MewsUrls.hub, labelId: 'nav-hub'),
+  (url: MewsUrls.posts, labelId: 'nav-posts'),
   (url: MewsUrls.bookmarks, labelId: 'nav-bookmarks'),
   (url: MewsUrls.subscriptions, labelId: 'nav-subscriptions'),
   (url: MewsUrls.history, labelId: 'nav-history'),
@@ -61,18 +67,59 @@ MewsListItem itemRow({
   required String label,
   String? date,
   String? note,
+  bool strong = false,
+  List<Choice> actions = const [],
 }) {
+  final text = date == null ? label : '$date $label';
   return MewsListItem([
     ParagraphBlock([
-      MewsLink(href, [MewsText(date == null ? label : '$date $label')]),
+      MewsLink(href, [
+        if (strong)
+          MewsEmphasis(EmphasisStyle.strong, [MewsText(text)])
+        else
+          MewsText(text),
+      ]),
       if (note != null) ...[
         const MewsLineBreak(),
         MewsEmphasis(EmphasisStyle.emphasis, [MewsText(note)]),
       ],
+      if (actions.isNotEmpty) ...[
+        const MewsLineBreak(),
+        ..._separated(
+            [for (final action in actions) MewsLink(action.href, [MewsText(action.label)])]),
+      ],
     ]),
   ]);
 }
 
+/// Joins runs with the separator between them, never around them.
+List<MewsInline> _separated(List<MewsInline> runs) {
+  final out = <MewsInline>[];
+  for (final run in runs) {
+    if (out.isNotEmpty) out.add(const MewsText(actionSeparator));
+    out.add(run);
+  }
+  return out;
+}
+
+/// A group of mutually exclusive values on one line, for a short group that
+/// would waste a list. The chosen value is plain text, the rest are links.
+List<MewsBlock> choiceLine({required String label, required List<Choice> options}) {
+  return [
+    ParagraphBlock([
+      MewsEmphasis(EmphasisStyle.emphasis, [MewsText(label)]),
+      const MewsText(actionSeparator),
+      ..._separated([
+        for (final option in options)
+          if (option.current)
+            MewsEmphasis(EmphasisStyle.strong, [MewsText(option.label)])
+          else
+            MewsLink(option.href, [MewsText(option.label)]),
+      ]),
+    ]),
+  ];
+}
+
 /// A group of mutually exclusive values: the current one plain, the rest links.
 ListBlock choiceRow(List<Choice> options) {
   return ListBlock(