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

html/content.css

@@ -1,6 +1,11 @@
+* {
+  box-sizing: border-box;
+}
+
 body {
   font-family: sans-serif;
-  margin: 1em 2em;
+  margin: 0;
+  padding: 0;
   line-height: 1.6;
   color: #222;
   background: #fff;
@@ -11,8 +16,86 @@ body {
     color: #ddd;
     background: #1e1e1e;
   }
+  a {
+    color: #78aeed;
+  }
+  img {
+    opacity: 0.85;
+  }
 }
 
-h1 {
-  font-size: 1.4em;
+#header {
+  padding: 1.5em 2em 1em;
+  border-bottom: 1px solid rgba(0,0,0,0.1);
+  margin-bottom: 1em;
+}
+
+@media (prefers-color-scheme: dark) {
+  #header {
+    border-bottom-color: rgba(255,255,255,0.1);
+  }
+}
+
+#feed-title {
+  font-size: 0.8em;
+  opacity: 0.6;
+  margin-bottom: 0.25em;
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+}
+
+#title {
+  font-size: 1.5em;
+  margin: 0 0 0.5em;
+  line-height: 1.3;
+}
+
+#meta {
+  font-size: 0.85em;
+  opacity: 0.6;
+  margin-bottom: 0.5em;
+}
+
+#meta span + span::before {
+  content: ' · ';
+}
+
+#link {
+  font-size: 0.85em;
+}
+
+#content {
+  padding: 0 2em 2em;
+  max-width: 800px;
+}
+
+#content img {
+  max-width: 100%;
+  height: auto;
+}
+
+#content pre {
+  overflow-x: auto;
+  background: rgba(0,0,0,0.05);
+  padding: 1em;
+  border-radius: 4px;
+}
+
+@media (prefers-color-scheme: dark) {
+  #content pre {
+    background: rgba(255,255,255,0.05);
+  }
+}
+
+#content blockquote {
+  border-left: 3px solid rgba(0,0,0,0.2);
+  margin-left: 0;
+  padding-left: 1em;
+  opacity: 0.8;
+}
+
+@media (prefers-color-scheme: dark) {
+  #content blockquote {
+    border-left-color: rgba(255,255,255,0.2);
+  }
 }