ui: overhaul sidebar, add content filters and state improvements

Sidebar layout:
- Replace AdwNavigationSplitView with GtkPaned for a resizable sidebar
  with a persistent width stored in GSettings.
- Apply navigation-sidebar CSS class to the content Stack only (not the
  ToolbarView) so both header bars share the same colour and height.
- Override Adwaita's automatic paned-first-child header tint and gap via
  application-level CSS.
- Remove the gap between the sidebar header and the first list item.
- Add toggle-sidebar button and F9 shortcut; sidebar visibility and width
  are persisted across restarts.

Loading indicator:
- Replace the large AdwSpinner status page + header Stack with a small
  Gtk.Spinner (16×16) in the header Stack so the header height never
  changes during loading.

Article row:
- Add hexpand to title and excerpt labels so text reflows when the
  sidebar is resized.

Content:
- Inline CSS into the HTML template at load time (/*INJECT_CSS*/
  placeholder) so WebKit does not need a custom URI scheme handler.
- Fix max-width centering and padding for article body and header.
- Fix embedded video/iframe auto-opening in browser by checking
  NavigationType::LinkClicked instead of is_user_gesture().

Content filters:
- Add Preferences dialog with a TextView for content-rewrite rules
  stored in GSettings (content-filters key).
- Rule format: "domain find replace [find replace …]" one per line.
- Rules are applied to article HTML before display and reloaded on
  every refresh.

Shortcuts:
- Add Ctrl+W to close, Ctrl+Q to quit, F1 for keyboard shortcuts
  overlay, j/k and arrow-key navigation via a capture-phase controller
  so keys work regardless of which widget has focus.

Misc:
- Set window title to "FeedTheMonkey" (fixes Hyprland title bar).
- Update About dialog website URL.

data/net.jeena.FeedTheMonkey.gschema.xml

@@ -21,5 +21,9 @@
       <default>1.0</default>
       <summary>WebView zoom level</summary>
     </key>
+    <key name="content-filters" type="s">
+      <default>''</default>
+      <summary>Content rewrite rules, one per line: domain from to [from to …]</summary>
+    </key>
   </schema>
 </schemalist>