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.

html/content.css

@@ -1,101 +1,131 @@
-* {
-  box-sizing: border-box;
+/* CSS custom properties are set from Rust via AdwStyleManager.
+   The :root defaults below act as a light-mode fallback only. */
+
+:root {
+  --bg:          #ffffff;
+  --fg:          #1a1a1a;
+  --fg-dim:      rgba(0,0,0,0.55);
+  --border:      rgba(0,0,0,0.12);
+  --header-bg:   #f6f5f4;
+  --link:        #1c71d8;
+  --code-bg:     rgba(0,0,0,0.06);
+  --blockquote-border: rgba(0,0,0,0.2);
+  --font:        sans-serif;
+  --font-size:   15px;
 }
 
-body {
-  font-family: sans-serif;
+:root[data-dark="1"] {
+  --bg:          #1e1e1e;
+  --fg:          rgba(255,255,255,0.87);
+  --fg-dim:      rgba(255,255,255,0.5);
+  --border:      rgba(255,255,255,0.12);
+  --header-bg:   #242424;
+  --link:        #78aeed;
+  --code-bg:     rgba(255,255,255,0.06);
+  --blockquote-border: rgba(255,255,255,0.2);
+}
+
+* { box-sizing: border-box; }
+
+html, body {
   margin: 0;
   padding: 0;
-  line-height: 1.6;
-  color: #222;
-  background: #fff;
+  background: var(--bg);
+  color: var(--fg);
+  font-family: var(--font);
+  font-size: var(--font-size);
+  word-wrap: break-word;
 }
 
-@media (prefers-color-scheme: dark) {
-  body {
-    color: #ddd;
-    background: #1e1e1e;
-  }
-  a {
-    color: #78aeed;
-  }
-  img {
-    opacity: 0.85;
-  }
+a {
+  color: var(--link);
+  text-decoration: none;
 }
 
-#header {
+article a {
+  text-decoration: underline;
+}
+
+header {
   padding: 1.5em 2em 1em;
-  border-bottom: 1px solid rgba(0,0,0,0.1);
-  margin-bottom: 1em;
+  background: var(--header-bg);
+  border-bottom: 1px solid var(--border);
 }
 
-@media (prefers-color-scheme: dark) {
-  #header {
-    border-bottom-color: rgba(255,255,255,0.1);
-  }
+header > .inner,
+article {
+  max-width: 720px;
+  margin-left: auto;
+  margin-right: auto;
 }
 
-#feed-title {
-  font-size: 0.8em;
-  opacity: 0.6;
-  margin-bottom: 0.25em;
-  text-transform: uppercase;
-  letter-spacing: 0.05em;
+header > .inner {
+  padding: 0 2em;
 }
 
-#title {
-  font-size: 1.5em;
-  margin: 0 0 0.5em;
+header h1 {
+  font-size: 1.3em;
+  margin: 0.2em 0 0.4em;
+  padding: 0;
   line-height: 1.3;
 }
 
-#meta {
-  font-size: 0.85em;
-  opacity: 0.6;
-  margin-bottom: 0.5em;
+header h1 a {
+  color: var(--fg);
 }
 
-#meta span + span::before {
-  content: ' · ';
-}
-
-#link {
+header p {
+  color: var(--fg-dim);
+  margin: 0;
+  padding: 0;
   font-size: 0.85em;
 }
 
-#content {
-  padding: 0 2em 2em;
-  max-width: 800px;
+article {
+  line-height: 1.6;
+  padding: 1.5em 2em 2em;
 }
 
-#content img {
+img {
   max-width: 100%;
   height: auto;
 }
 
-#content pre {
-  overflow-x: auto;
-  background: rgba(0,0,0,0.05);
+div > a:only-child img,
+figure > a:only-child img,
+p > a:only-child img,
+figure > img:only-child,
+div > img:only-child,
+p > img:only-child {
+  display: block;
+  margin: 1em auto;
+  float: none !important;
+}
+
+pre {
+  overflow: auto;
+  background: var(--code-bg);
   padding: 1em;
-  border-radius: 4px;
+  border-radius: 6px;
+  font-size: 0.9em;
 }
 
-@media (prefers-color-scheme: dark) {
-  #content pre {
-    background: rgba(255,255,255,0.05);
-  }
+code {
+  background: var(--code-bg);
+  padding: 0.15em 0.35em;
+  border-radius: 3px;
+  font-size: 0.9em;
 }
 
-#content blockquote {
-  border-left: 3px solid rgba(0,0,0,0.2);
+pre code {
+  background: none;
+  padding: 0;
+}
+
+blockquote {
+  border-left: 3px solid var(--blockquote-border);
   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);
-  }
+  color: var(--fg-dim);
+  font-style: italic;
 }