feat(ui): Phase 7 C3 — Priority 过滤 + 消息详情面板

- sidebar FILTERS 加 Max/High/Normal/Low 四档（dot 色与 priority 配色一致），点击切换 priority-filter（0=All, 1=Low, 2=Normal, 3=High, 4=Max）
- MessageList 加 priority-filter property，通过 visible 谓词按 Slint Priority enum 匹配；保留原 idx 让 selected-index 不随 filter 错位
- 右栏加 detail panel：app + priority dot、title、id、relative_time、body、has_click_url 时显示 Open link 按钮（复用 C2 的 message-card-clicked 走 url::open）
- detail panel 与消息列表水平分割，selected-message-index < 0 时自动隐藏；切换 Settings/About 时不显示

ui/message-list.slint

@@ -5,13 +5,26 @@
 // the dependency footprint minimal until Phase 6 introduces the real
 // std-widgets-backed ListView (which requires an explicit `import` from
 // `@std-widgets.slint` and an `include_paths` config in build.rs).
+//
+// Phase 7 C3: a `priority-filter` property hides non-matching cards via
+// `visible`. Hiding (vs filtering the model) keeps the original index
+// intact, which lets `selected-index` stay correct when the filter
+// changes — clicking item 5 with the filter still applies to row 5
+// of the underlying list, not the post-filter row.
 
-import { MessageItem, MessageCard } from "./message-card.slint";
+import { MessageItem, MessageCard, Priority } from "./message-card.slint";
 
 export component MessageList inherits Rectangle {
     in property <[MessageItem]> messages;
     in-out property <int> selected-index: -1;
 
+    // 0 = show all (no filter). 1 = Priority::Low, 2 = Normal,
+    // 3 = High, 4 = Max. Off-by-one alignment with the Slint
+    // `Priority` enum (Low=0, Normal=1, High=2, Max=3) is
+    // intentional so the UI shows "All / Low / Normal / High / Max"
+    // while the wire-level enum keeps its 0..3 range.
+    in property <int> priority-filter: 0;
+
     in property <color> card-border: #3a3b40;
 
     callback message-clicked(int);
@@ -24,6 +37,16 @@ export component MessageList inherits Rectangle {
         for item[idx] in messages: MessageCard {
             item: item;
             selected: idx == root.selected-index;
+            // Show every card when filter is "All"; otherwise only
+            // the priority bucket the user picked. Slint lets us
+            // compare an enum directly against an int literal, so
+            // `item.priority == 0` matches Priority::Low without
+            // any explicit conversion.
+            visible: root.priority-filter == 0
+                || (root.priority-filter == 1 && item.priority == Priority.Low)
+                || (root.priority-filter == 2 && item.priority == Priority.Normal)
+                || (root.priority-filter == 3 && item.priority == Priority.High)
+                || (root.priority-filter == 4 && item.priority == Priority.Max);
             card-border: card-border;
             clicked => {
                 root.selected-index = idx;