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.

src/preferences_dialog.rs

@@ -0,0 +1,75 @@
+use gtk4::prelude::*;
+use gtk4::subclass::prelude::*;
+use gtk4::{gio, glib};
+
+glib::wrapper! {
+    pub struct PreferencesDialog(ObjectSubclass<imp::PreferencesDialog>)
+        @extends libadwaita::Dialog, gtk4::Widget,
+        @implements gtk4::Accessible, gtk4::Buildable, gtk4::ConstraintTarget;
+}
+
+impl PreferencesDialog {
+    pub fn new() -> Self {
+        glib::Object::builder().build()
+    }
+}
+
+pub mod imp {
+    use super::*;
+    use gtk4::CompositeTemplate;
+    use libadwaita::subclass::prelude::*;
+
+    #[derive(CompositeTemplate, Default)]
+    #[template(resource = "/net/jeena/FeedTheMonkey/ui/preferences_dialog.ui")]
+    pub struct PreferencesDialog {
+        #[template_child]
+        pub filters_text_view: TemplateChild<gtk4::TextView>,
+    }
+
+    #[glib::object_subclass]
+    impl ObjectSubclass for PreferencesDialog {
+        const NAME: &'static str = "PreferencesDialog";
+        type Type = super::PreferencesDialog;
+        type ParentType = libadwaita::Dialog;
+
+        fn class_init(klass: &mut Self::Class) {
+            klass.bind_template();
+        }
+
+        fn instance_init(obj: &glib::subclass::InitializingObject<Self>) {
+            obj.init_template();
+        }
+    }
+
+    impl ObjectImpl for PreferencesDialog {
+        fn constructed(&self) {
+            self.parent_constructed();
+            self.load();
+            // Save on every text change
+            let obj_weak = self.obj().downgrade();
+            self.filters_text_view.buffer().connect_changed(move |_| {
+                if let Some(obj) = obj_weak.upgrade() {
+                    obj.imp().save();
+                }
+            });
+        }
+    }
+
+    impl PreferencesDialog {
+        fn load(&self) {
+            let settings = gio::Settings::new("net.jeena.FeedTheMonkey");
+            let text = settings.string("content-filters");
+            self.filters_text_view.buffer().set_text(&text);
+        }
+
+        fn save(&self) {
+            let buf = self.filters_text_view.buffer();
+            let text = buf.text(&buf.start_iter(), &buf.end_iter(), false);
+            let settings = gio::Settings::new("net.jeena.FeedTheMonkey");
+            settings.set_string("content-filters", &text).ok();
+        }
+    }
+
+    impl WidgetImpl for PreferencesDialog {}
+    impl AdwDialogImpl for PreferencesDialog {}
+}