app: implement Epics 2–10

Add the full application logic on top of the Epic 1 skeleton:

Epic 2 — Authentication
- LoginDialog (AdwDialog, Blueprint template) with server URL,
  username, and password fields; emits logged-in signal on submit
- credentials.rs: store/load/clear via libsecret (password_store_sync /
  password_search_sync / password_clear_sync, v0_19 feature)
- api.rs: Api::login() parses Auth= token from ClientLogin response;
  fetch_write_token() fetches the write token
- Auto-login on startup from stored credentials; logout with
  AdwAlertDialog confirmation; login errors shown in AdwAlertDialog

Epic 3 — Article fetching
- model.rs: Article struct and ArticleObject GObject wrapper with
  unread property for list store binding
- Api::fetch_unread() deserializes Google Reader JSON, derives unread
  from categories, generates plain-text excerpt
- Sidebar uses a GtkStack with placeholder / loading / empty / error /
  list pages; AdwSpinnerPaintable while fetching; Try Again button

Epic 4 — Sidebar
- article_row.blp: composite template with feed title, date, title,
  and excerpt labels
- ArticleRow GObject subclass; binds ArticleObject, watches unread
  notify to apply .dim-label on the title; relative timestamp format

Epic 5 — Content pane
- content.html updated: setArticle(), checkKey(), feedthemonkey: URI
  navigation scheme; dark mode via prefers-color-scheme
- content.css: proper article layout, dark mode, code blocks
- WebView loaded from GResource; decide-policy intercepts
  feedthemonkey:{next,previous,open} and all external links

Epic 6 — Read state
- Api::mark_read() / mark_unread() via edit-tag endpoint
- Optimistic unread toggle on ArticleObject; background API calls;
  mark_unread_guard prevents re-marking on navigation
- AdwToast shown on mark-unread

Epic 7 — Keyboard shortcuts
- GtkShortcutController on window for all shortcuts from the backlog
- shortcuts.blp: AdwShortcutsWindow documenting all shortcuts
- F1 opens shortcuts dialog; Ctrl+W closes window; Ctrl+Q quits

Epic 8 — Zoom
- zoom_in/zoom_out/zoom_reset wired to Ctrl+±/0; zoom level saved to
  and restored from GSettings zoom-level key

Epic 9 — Window state persistence
- Window width/height/maximized saved on close, restored on open
- (Sidebar width deferred — AdwNavigationSplitView fraction binding)

Epic 10 — Polish
- AdwAboutDialog with app name, version, GPL-3.0, website
- Logout confirmation AdwAlertDialog with destructive button
- Win.toggle-fullscreen action (F11)
- Api dropped on window close to cancel in-flight requests

src/login_dialog.rs

@@ -0,0 +1,105 @@
+use gtk4::glib;
+
+glib::wrapper! {
+    pub struct LoginDialog(ObjectSubclass<imp::LoginDialog>)
+        @extends libadwaita::Dialog, gtk4::Widget,
+        @implements gtk4::Accessible, gtk4::Buildable, gtk4::ConstraintTarget;
+}
+
+impl LoginDialog {
+    pub fn new() -> Self {
+        glib::Object::new()
+    }
+}
+
+mod imp {
+    use super::*;
+    use gtk4::prelude::*;
+    use gtk4::subclass::prelude::*;
+    use gtk4::CompositeTemplate;
+    use libadwaita::prelude::*;
+    use libadwaita::subclass::prelude::*;
+
+    #[derive(CompositeTemplate, Default)]
+    #[template(resource = "/net/jeena/FeedTheMonkey/ui/login_dialog.ui")]
+    pub struct LoginDialog {
+        #[template_child]
+        pub server_url_row: TemplateChild<libadwaita::EntryRow>,
+        #[template_child]
+        pub username_row: TemplateChild<libadwaita::EntryRow>,
+        #[template_child]
+        pub password_row: TemplateChild<libadwaita::PasswordEntryRow>,
+        #[template_child]
+        pub login_button: TemplateChild<libadwaita::ButtonRow>,
+    }
+
+    #[glib::object_subclass]
+    impl ObjectSubclass for LoginDialog {
+        const NAME: &'static str = "LoginDialog";
+        type Type = super::LoginDialog;
+        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 LoginDialog {
+        fn signals() -> &'static [glib::subclass::Signal] {
+            use std::sync::OnceLock;
+            static SIGNALS: OnceLock<Vec<glib::subclass::Signal>> = OnceLock::new();
+            SIGNALS.get_or_init(|| {
+                vec![
+                    glib::subclass::Signal::builder("logged-in")
+                        .param_types([
+                            String::static_type(),
+                            String::static_type(),
+                            String::static_type(),
+                        ])
+                        .build(),
+                ]
+            })
+        }
+
+        fn constructed(&self) {
+            self.parent_constructed();
+
+            let obj_weak = self.obj().downgrade();
+            self.login_button.connect_activated(move |_| {
+                if let Some(dialog) = obj_weak.upgrade() {
+                    dialog.imp().on_login_clicked();
+                }
+            });
+
+            // Also trigger on Enter in password row
+            let obj_weak2 = self.obj().downgrade();
+            self.password_row.connect_apply(move |_| {
+                if let Some(dialog) = obj_weak2.upgrade() {
+                    dialog.imp().on_login_clicked();
+                }
+            });
+        }
+    }
+
+    impl LoginDialog {
+        fn on_login_clicked(&self) {
+            let server_url = self.server_url_row.text().trim().to_string();
+            let username = self.username_row.text().trim().to_string();
+            let password = self.password_row.text().to_string();
+
+            if server_url.is_empty() || username.is_empty() || password.is_empty() {
+                return;
+            }
+
+            self.obj().close();
+            self.obj().emit_by_name::<()>("logged-in", &[&server_url, &username, &password]);
+        }
+    }
+
+    impl WidgetImpl for LoginDialog {}
+    impl AdwDialogImpl for LoginDialog {}
+}