Add tags, export/import, and changelog features

- Tag editor in detail view with add/remove pill chips
- Tag filter chips in library view for filtering by tag
- Shared backup module for app list export/import (JSON v2)
- CLI export/import refactored to use shared module
- GUI export/import via file picker dialogs in hamburger menu
- GitHub release history enrichment for catalog apps
- Changelog preview in updates view with expandable rows
- DB migration v19 for catalog release_history column

src/ui/updates_view.rs

@@ -262,11 +262,6 @@ fn populate_update_list(state: &Rc<UpdatesState>) {
     for record in &updatable {
         let name = record.app_name.as_deref().unwrap_or(&record.filename);
 
-        let row = adw::ActionRow::builder()
-            .title(name)
-            .activatable(false)
-            .build();
-
         // Show version info: current -> latest (with size if available)
         let current = record.app_version.as_deref().unwrap_or("unknown");
         let latest = record.latest_version.as_deref().unwrap_or("unknown");
@@ -275,12 +270,46 @@ fn populate_update_list(state: &Rc<UpdatesState>) {
         } else {
             format!("{} -> {}", current, latest)
         };
-        row.set_subtitle(&subtitle);
+
+        // Try to find changelog from the app's own release_history or from catalog
+        let changelog = find_changelog_for_version(
+            &state.db, record, latest,
+        );
+
+        // Use ExpanderRow if we have changelog, otherwise plain ActionRow
+        let row: adw::ExpanderRow = adw::ExpanderRow::builder()
+            .title(name)
+            .subtitle(&subtitle)
+            .show_enable_switch(false)
+            .expanded(false)
+            .build();
 
         // App icon
         let icon = widgets::app_icon(record.icon_path.as_deref(), name, 32);
         row.add_prefix(&icon);
 
+        // "What's new" content inside the expander
+        let changelog_text = match &changelog {
+            Some(text) => text.clone(),
+            None => i18n("Release notes not available"),
+        };
+        let changelog_label = gtk::Label::builder()
+            .label(&changelog_text)
+            .wrap(true)
+            .xalign(0.0)
+            .css_classes(if changelog.is_some() { vec!["body"] } else { vec!["dim-label", "caption"] })
+            .margin_start(12)
+            .margin_end(12)
+            .margin_top(8)
+            .margin_bottom(8)
+            .selectable(true)
+            .build();
+        let changelog_row = adw::ActionRow::builder()
+            .activatable(false)
+            .child(&changelog_label)
+            .build();
+        row.add_row(&changelog_row);
+
         // Individual update button
         let update_btn = gtk::Button::builder()
             .icon_name("software-update-available-symbolic")
@@ -341,3 +370,81 @@ fn populate_update_list(state: &Rc<UpdatesState>) {
         state.list_box.append(&row);
     }
 }
+
+/// Try to find changelog/release notes for a specific version.
+/// Checks the installed app's release_history first, then falls back
+/// to the catalog app's release_history (populated by GitHub enrichment).
+fn find_changelog_for_version(
+    db: &Database,
+    record: &crate::core::database::AppImageRecord,
+    target_version: &str,
+) -> Option<String> {
+    // First check the installed app's own release_history
+    if let Some(ref history_json) = record.release_history {
+        if let Some(text) = extract_version_notes(history_json, target_version) {
+            return Some(text);
+        }
+    }
+
+    // Fall back to catalog app's release_history
+    let app_name = record.app_name.as_deref().unwrap_or(&record.filename);
+    if let Ok(Some(ref history_json)) = db.get_catalog_release_history_by_name(app_name) {
+        if let Some(text) = extract_version_notes(history_json, target_version) {
+            return Some(text);
+        }
+    }
+
+    None
+}
+
+/// Parse release_history JSON and extract the description for a given version.
+/// Format: [{"version": "1.0.0", "date": "2026-01-01", "description": "..."}]
+fn extract_version_notes(history_json: &str, target_version: &str) -> Option<String> {
+    let releases: Vec<serde_json::Value> = serde_json::from_str(history_json).ok()?;
+
+    // Try exact match first
+    for release in &releases {
+        if let Some(ver) = release.get("version").and_then(|v| v.as_str()) {
+            if ver == target_version {
+                return release.get("description")
+                    .and_then(|d| d.as_str())
+                    .map(|s| truncate_changelog(s));
+            }
+        }
+    }
+
+    // If no exact match, try matching without "v" prefix on both sides
+    let clean_target = target_version.strip_prefix('v').unwrap_or(target_version);
+    for release in &releases {
+        if let Some(ver) = release.get("version").and_then(|v| v.as_str()) {
+            let clean_ver = ver.strip_prefix('v').unwrap_or(ver);
+            if clean_ver == clean_target {
+                return release.get("description")
+                    .and_then(|d| d.as_str())
+                    .map(|s| truncate_changelog(s));
+            }
+        }
+    }
+
+    // No matching version found - show the latest entry's notes as a fallback
+    // (the first entry is typically the newest release)
+    if let Some(first) = releases.first() {
+        if let Some(desc) = first.get("description").and_then(|d| d.as_str()) {
+            let ver = first.get("version").and_then(|v| v.as_str()).unwrap_or("?");
+            return Some(format!("(v{}) {}", ver, truncate_changelog(desc)));
+        }
+    }
+
+    None
+}
+
+/// Truncate long changelog text to keep the UI compact.
+fn truncate_changelog(text: &str) -> String {
+    let max_len = 500;
+    let trimmed = text.trim();
+    if trimmed.len() <= max_len {
+        trimmed.to_string()
+    } else {
+        format!("{}...", &trimmed[..max_len])
+    }
+}