feat: implement command autocomplete for journal input

Refactor journal input command parsing into a dedicated module and
add a `CompletionsDropdown` component to provide real-time suggestions
for commands (/roll, /spark, /link, /stat) and their arguments.

src/components/journal/command-parser.ts

@@ -0,0 +1,83 @@
+/**
+ * Command parser — parse a raw input string into a typed ParsedInput.
+ *
+ * Pure function with no component dependencies, so it can live in its own
+ * module and keep JournalInput small.
+ */
+
+export interface CompletionItem {
+  label: string;
+  kind: "command" | "value" | "no-results";
+  insertText: string;
+}
+
+export interface ParsedInput {
+  type: "chat" | "roll" | "spark" | "link" | "stat";
+  payload: Record<string, unknown>;
+  error?: string;
+}
+
+export function parseInput(raw: string): ParsedInput {
+  if (raw.startsWith("/roll ")) {
+    const notation = raw.slice("/roll ".length).trim();
+    if (!notation)
+      return { type: "roll", payload: {}, error: "需要骰子表达式" };
+    return { type: "roll", payload: { notation, label: notation } };
+  }
+
+  if (raw.startsWith("/spark ")) {
+    const key = raw.slice("/spark ".length).trim();
+    if (!key) return { type: "spark", payload: {}, error: "需要种子表键名" };
+    return { type: "spark", payload: { key } };
+  }
+
+  if (raw.startsWith("/link ")) {
+    const arg = raw.slice("/link ".length).trim();
+    if (!arg) return { type: "link", payload: {}, error: "需要路径" };
+    const hashIdx = arg.indexOf("#");
+    const path = hashIdx === -1 ? arg : arg.slice(0, hashIdx);
+    const section =
+      hashIdx === -1 ? undefined : arg.slice(hashIdx + 1) || undefined;
+    return { type: "link", payload: { path, section } };
+  }
+
+  if (raw.startsWith("/stat ")) {
+    const arg = raw.slice("/stat ".length).trim();
+    if (!arg)
+      return { type: "stat", payload: {}, error: "需要 stat 命令 (set/del/roll)" };
+
+    if (arg.startsWith("set ")) {
+      const rest = arg.slice("set ".length).trim();
+      const eqIdx = rest.indexOf("=");
+      if (eqIdx === -1)
+        return { type: "stat", payload: {}, error: "格式: /stat set key=value" };
+      const key = rest.slice(0, eqIdx).trim();
+      const value = rest.slice(eqIdx + 1).trim();
+      if (!key || !value)
+        return { type: "stat", payload: {}, error: "key 和 value 不能为空" };
+      return { type: "stat", payload: { action: "set", key, value } };
+    }
+
+    if (arg.startsWith("del ")) {
+      const key = arg.slice("del ".length).trim();
+      if (!key)
+        return { type: "stat", payload: {}, error: "格式: /stat del key" };
+      return { type: "stat", payload: { action: "del", key } };
+    }
+
+    if (arg.startsWith("roll ")) {
+      const key = arg.slice("roll ".length).trim();
+      if (!key)
+        return { type: "stat", payload: {}, error: "格式: /stat roll key" };
+      return { type: "stat", payload: { action: "roll", key } };
+    }
+
+    return { type: "stat", payload: {}, error: "未知 stat 子命令: set/del/roll" };
+  }
+
+  if (raw === "/roll" || raw === "/spark" || raw === "/link" || raw === "/stat") {
+    return { type: "chat", payload: {}, error: "请补全命令" };
+  }
+
+  return { type: "chat", payload: { text: raw } };
+}