Add AppImageHub.com OCS API as primary catalog source

Integrate the Pling/OCS REST API (appimagehub.com) as the primary catalog
source with richer metadata than the existing appimage.github.io feed.

Backend:
- Add OCS API fetch with pagination, lenient JSON deserializers for loosely
  typed numeric fields, and non-AppImage file filtering (.dmg, .exe, etc.)
- Database migration v17 adds OCS-specific columns (ocs_id, downloads, score,
  typename, personid, description, summary, version, tags, etc.)
- Deduplicate secondary source apps against OCS entries
- Shrink OCS CDN icon URLs from 770x540 to 100x100 for faster loading
- Clear stale screenshot and icon caches on sync
- Extract GitHub repo links from OCS HTML descriptions
- Add fetch_ocs_download_files() to get all version files for an app
- Resolve fresh JWT download URLs per slot at install time

Detail page:
- Fetch OCS download files on page open and populate install SplitButton
  with version dropdown (newest first, filtered for AppImage only)
- Show OCS metadata: downloads, score, author, typename, tags, comments,
  created/updated dates, architecture, filename, file size, MD5
- Prefer ocs_description (full HTML with features/changelog) over short
  summary for the About section
- Add html_to_description() to preserve formatting (lists, paragraphs)
- Remove redundant Download link from Links section
- Escape ampersands in Pango markup subtitles (categories, typename, tags)

Catalog view:
- OCS source syncs first as primary, appimage.github.io as secondary
- Featured apps consider OCS download counts alongside GitHub stars

UI:
- Add pulldown-cmark for GitHub README markdown rendering in detail pages
- Add build_markdown_view() widget for rendered markdown content

src/ui/catalog_tile.rs

@@ -45,9 +45,11 @@ pub fn build_catalog_tile(app: &CatalogApp) -> gtk::FlowBoxChild {
         .build();
     inner.append(&name_label);
 
-    // Description (always 2 lines for uniform height)
-    let plain = app.description.as_deref()
+    // Description (always 2 lines for uniform height) - prefer OCS summary, then GitHub description
+    let plain = app.ocs_summary.as_deref()
         .filter(|d| !d.is_empty())
+        .or(app.github_description.as_deref().filter(|d| !d.is_empty()))
+        .or(app.description.as_deref().filter(|d| !d.is_empty()))
         .map(|d| strip_html(d))
         .unwrap_or_default();
     let snippet: String = plain.chars().take(80).collect();
@@ -139,6 +141,7 @@ pub fn build_catalog_tile(app: &CatalogApp) -> gtk::FlowBoxChild {
         .child(&card)
         .build();
     child.add_css_class("activatable");
+    widgets::set_pointer_cursor(&child);
 
     child
 }
@@ -158,6 +161,7 @@ pub fn build_featured_tile(app: &CatalogApp) -> gtk::Box {
     card.add_css_class("card");
     card.add_css_class("catalog-featured-card");
     card.add_css_class("activatable");
+    widgets::set_pointer_cursor(&card);
     card.set_widget_name(&format!("featured-{}", app.id));
 
     // Screenshot preview area (top)
@@ -212,27 +216,29 @@ pub fn build_featured_tile(app: &CatalogApp) -> gtk::Box {
         .build();
     text_box.append(&name_label);
 
-    // Description (1 line in featured since space is tight)
-    if let Some(ref desc) = app.description {
-        if !desc.is_empty() {
-            let plain = strip_html(desc);
-            let snippet: String = plain.chars().take(60).collect();
-            let text = if snippet.len() < plain.len() {
-                format!("{}...", snippet.trim_end())
-            } else {
-                snippet
-            };
-            let desc_label = gtk::Label::builder()
-                .label(&text)
-                .css_classes(["caption", "dim-label"])
-                .ellipsize(gtk::pango::EllipsizeMode::End)
-                .lines(1)
-                .xalign(0.0)
-                .max_width_chars(35)
-                .halign(gtk::Align::Start)
-                .build();
-            text_box.append(&desc_label);
-        }
+    // Description (1 line in featured since space is tight) - prefer OCS summary
+    let feat_desc = app.ocs_summary.as_deref()
+        .filter(|d| !d.is_empty())
+        .or(app.github_description.as_deref().filter(|d| !d.is_empty()))
+        .or(app.description.as_deref().filter(|d| !d.is_empty()));
+    if let Some(desc) = feat_desc {
+        let plain = strip_html(desc);
+        let snippet: String = plain.chars().take(60).collect();
+        let text = if snippet.len() < plain.len() {
+            format!("{}...", snippet.trim_end())
+        } else {
+            snippet
+        };
+        let desc_label = gtk::Label::builder()
+            .label(&text)
+            .css_classes(["caption", "dim-label"])
+            .ellipsize(gtk::pango::EllipsizeMode::End)
+            .lines(1)
+            .xalign(0.0)
+            .max_width_chars(35)
+            .halign(gtk::Align::Start)
+            .build();
+        text_box.append(&desc_label);
     }
 
     // Badge row: category + stars
@@ -273,7 +279,71 @@ pub fn build_featured_tile(app: &CatalogApp) -> gtk::Box {
     card
 }
 
-/// Strip HTML tags from a string, returning plain text.
+/// Convert HTML to readable formatted plain text, preserving paragraph breaks,
+/// line breaks, and list structure. Suitable for detail page descriptions.
+pub fn html_to_description(html: &str) -> String {
+    let mut result = String::with_capacity(html.len());
+    let mut in_tag = false;
+    let mut tag_buf = String::new();
+
+    for ch in html.chars() {
+        match ch {
+            '<' => {
+                in_tag = true;
+                tag_buf.clear();
+            }
+            '>' if in_tag => {
+                in_tag = false;
+                let tag = tag_buf.trim().to_lowercase();
+                let tag_name = tag.split_whitespace().next().unwrap_or("");
+                match tag_name {
+                    "br" | "br/" => result.push('\n'),
+                    "/p" => result.push_str("\n\n"),
+                    "li" => result.push_str("\n  - "),
+                    "/ul" | "/ol" => result.push('\n'),
+                    s if s.starts_with("/h") => result.push_str("\n\n"),
+                    _ => {}
+                }
+            }
+            _ if in_tag => tag_buf.push(ch),
+            _ => result.push(ch),
+        }
+    }
+
+    // Decode HTML entities
+    let decoded = result
+        .replace("&amp;", "&")
+        .replace("&lt;", "<")
+        .replace("&gt;", ">")
+        .replace("&quot;", "\"")
+        .replace("&#39;", "'")
+        .replace("&apos;", "'")
+        .replace("&nbsp;", " ");
+
+    // Clean up: trim lines, collapse multiple blank lines
+    let trimmed: Vec<&str> = decoded.lines().map(|l| l.trim()).collect();
+    let mut cleaned = String::new();
+    let mut prev_blank = false;
+    for line in &trimmed {
+        if line.is_empty() {
+            if !prev_blank && !cleaned.is_empty() {
+                cleaned.push('\n');
+                prev_blank = true;
+            }
+        } else {
+            if prev_blank {
+                cleaned.push('\n');
+            }
+            cleaned.push_str(line);
+            cleaned.push('\n');
+            prev_blank = false;
+        }
+    }
+
+    cleaned.trim().to_string()
+}
+
+/// Strip all HTML tags and collapse whitespace. Suitable for short descriptions on tiles.
 pub fn strip_html(html: &str) -> String {
     let mut result = String::with_capacity(html.len());
     let mut in_tag = false;