perf(dashboard): 用量统计从秒级降到感知不到 — SQLite 连接单例、配置读取缓存、聚合结果缓存、快照持久化 + 后台校验

功能与数据口径零变化（用量数字、字段、IPC 签名、导出格式均与 v0.8.1 一致）。

- db.rs：新增进程级连接单例（含迁移只跑一次），卡片墙场景由 200+ 次「开连接 + 7 条 DDL」降为 1 次；
  新增写路径代际计数器供各处读缓存判失效
- commands.rs：load_agent_config_command 结果缓存（db 代际 + config.toml 文件戳双重校验），
  N 张供应商卡片由 N 次全量加载（config.toml 解析 + SQLite 全量读 + 逐供应商 settings 读）降为 1 次
- dashboard.rs：
  * build_all_models 单遍轻量聚合，替换原先为取一张模型表而白算日/明细/供应商三份数据的整表遍历
  * SUMMARY_CACHE 按记录集 Arc 身份 + 日期 + 分钟桶缓存热力图/模型总览/各范围汇总，切范围来回近乎零成本
  * 归档会话列表与合成记录按代际 + Arc 身份缓存（合并后的 Arc 保持稳定，否则上层缓存永不命中）
  * 解析结果持久化到 ~/.kimi-switch/scan-cache-<hash>.json：重启后首次 get_summary 直读快照
    （实测 6.4 万条记录 / 1.03 GB：22~27s → 0.85s），随后后台线程全量校验，
    有变化时经 dashboard://records-updated 事件推送，前端无感刷新（先快后准）；
    快照损坏/指纹不符静默降级为全量扫描，refresh=true 语义不变
- useDashboard.ts：stale-while-revalidate，切范围/重进页面先呈现旧数据再静默刷新，不再闪加载态；
  订阅上述事件做无感更新，保留原世代号防竞态

测试：Rust 193 项（+16）、前端 130 项全绿

src/hooks/useDashboard.ts

@@ -1,35 +1,68 @@
 import { useCallback, useEffect, useRef, useState } from "react";
 import { invoke } from "@tauri-apps/api/core";
+import { listen } from "@tauri-apps/api/event";
 import type { SummaryResult } from "../types/dashboard";
 
 export type DashboardRange = "today" | "yesterday" | "7d" | "30d" | "all";
 
 const RANGE_STORAGE_KEY = "kimi-switch-dashboard-range";
 
-export function useDashboard() {
-  const [range, setRange] = useState<DashboardRange>(() => {
-    try {
-      const stored = localStorage.getItem(RANGE_STORAGE_KEY) as DashboardRange | null;
-      if (stored === "today" || stored === "yesterday" || stored === "7d" || stored === "30d" || stored === "all") {
-        return stored;
-      }
-    } catch {
-      // ignore
-    }
-    return "30d";
-  });
+/**
+ * Emitted by the backend when the background verification walk found records the
+ * fast (disk-cache) path did not have. The payload is empty — the hook only
+ * needs to know that a re-fetch is worth making.
+ */
+const RECORDS_UPDATED_EVENT = "dashboard://records-updated";
 
-  const [data, setData] = useState<SummaryResult | null>(null);
+const RANGES: readonly DashboardRange[] = ["today", "yesterday", "7d", "30d", "all"];
+
+function readStoredRange(): DashboardRange {
+  try {
+    const stored = localStorage.getItem(RANGE_STORAGE_KEY) as DashboardRange | null;
+    if (stored && (RANGES as readonly string[]).includes(stored)) {
+      return stored;
+    }
+  } catch {
+    // ignore
+  }
+  return "30d";
+}
+
+/**
+ * Last successful payload per range, kept across mounts so a range switch (and
+ * reopening the page) paints instantly from memory and revalidates in the
+ * background instead of showing a spinner over data that is already known.
+ * Bounded by the five ranges the dashboard offers.
+ */
+const summaryCache = new Map<DashboardRange, SummaryResult>();
+
+export function useDashboard() {
+  const [range, setRange] = useState<DashboardRange>(readStoredRange);
+
+  const [data, setData] = useState<SummaryResult | null>(
+    () => summaryCache.get(readStoredRange()) ?? null,
+  );
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState<string | null>(null);
   /** Round-trip timing for the last get_summary call (user-perceived lag). */
   const [loadStats, setLoadStats] = useState<{ ms: number } | null>(null);
   // Generation counter: stale responses (superseded range / unmounted) are dropped.
   const genRef = useRef(0);
+  // The event handler must call the *current* refresh, not the one captured when
+  // the listener was installed, so it reads the callback through a ref.
+  const refreshRef = useRef<(force?: boolean) => Promise<void>>(async () => {});
 
   const refresh = useCallback(async (force = false) => {
     const gen = ++genRef.current;
-    setLoading(true);
+    // Stale-while-revalidate: hand the cached payload for this range to the
+    // view before the request goes out, so a range switch is instant and the
+    // response only refreshes numbers that are already on screen. `loading` is
+    // raised only when there is nothing to show (first load of a range) or when
+    // the user asked for a refresh — the button keeps its busy state, the silent
+    // revalidation behind a cached range does not.
+    const cached = summaryCache.get(range);
+    if (cached) setData(cached);
+    setLoading(force || !cached);
     setError(null);
     const start = performance.now();
     try {
@@ -38,6 +71,7 @@ export function useDashboard() {
         refresh: force,
       });
       if (genRef.current !== gen) return;
+      summaryCache.set(range, result);
       setData(result);
       setLoadStats({ ms: Math.round(performance.now() - start) });
     } catch (err) {
@@ -49,10 +83,39 @@ export function useDashboard() {
     }
   }, [range]);
 
+  refreshRef.current = refresh;
+
   useEffect(() => {
     refresh();
   }, [refresh]);
 
+  // The first load of a session answers from the on-disk snapshot, which can be
+  // behind a session written since the app last ran. The backend verifies in the
+  // background and signals here when the numbers moved; re-fetching silently
+  // (never raising `loading`) keeps the already-drawn dashboard on screen and
+  // just corrects it. A silent call still takes a fresh generation, so it cannot
+  // be interleaved with a range switch in flight.
+  useEffect(() => {
+    let unlisten: (() => void) | undefined;
+    let cancelled = false;
+    listen(RECORDS_UPDATED_EVENT, () => {
+      refreshRef.current();
+    })
+      .then((fn) => {
+        // The effect may have been torn down before `listen` resolved.
+        if (cancelled) fn();
+        else unlisten = fn;
+      })
+      .catch(() => {
+        // No event bridge (e.g. a browser build): the dashboard still works,
+        // it just will not be corrected mid-session.
+      });
+    return () => {
+      cancelled = true;
+      unlisten?.();
+    };
+  }, []);
+
   // Ignore late responses after unmount.
   useEffect(() => {
     return () => {