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

data/ui/article_row.blp

@@ -0,0 +1,42 @@
+using Gtk 4.0;
+using Adw 1;
+
+template $ArticleRow : Gtk.Box {
+  orientation: vertical;
+  margin-top: 12;
+  margin-bottom: 12;
+  margin-start: 12;
+  margin-end: 12;
+  spacing: 4;
+
+  Box {
+    orientation: horizontal;
+    spacing: 4;
+
+    Label feed_title_label {
+      hexpand: true;
+      xalign: 0;
+      ellipsize: end;
+      styles ["dim-label", "caption"]
+    }
+
+    Label date_label {
+      xalign: 1;
+      styles ["dim-label", "caption"]
+    }
+  }
+
+  Label title_label {
+    xalign: 0;
+    wrap: true;
+    lines: 2;
+    ellipsize: end;
+  }
+
+  Label excerpt_label {
+    xalign: 0;
+    ellipsize: end;
+    lines: 1;
+    styles ["dim-label", "caption"]
+  }
+}