Drop the last two timers: prefetch and the UI stats poll go event-driven

Phase 2.5 — `prefetch_loop` no longer wakes every 30s. It subscribes to
`MailStore::subscribe()` and only catches up missing bodies when the
generation counter ticks (event-bus delta, sync_folder finishing,
bootstrap). When the store is quiet — every cached mail has its .eml on
disk — the loop sleeps indefinitely on the watch channel. The
`PREFETCH_INTERVAL` constant is gone.

Phase 2.6 — the dashboard 1s `setInterval` is gone too. `BridgeHandle`
gets a `stats_dirty_tx: broadcast::Sender<()>` that pulses on every
`MailStore` bump, every `WsState` transition, and the start / stop
status transitions. A small watcher task inside `start()` plumbs the
two `watch::Receiver`s into the broadcast. The Tauri layer
(`stream_stats`) subscribes via `BridgeHandle::subscribe_stats()`,
takes a stats + status snapshot on every pulse (and once at startup),
and emits two events `bridge://stats` / `bridge://status` to the
webview. The React hook replaces `setInterval(refresh, 1000)` with two
`listen()` subscriptions; the initial `refresh()` still seeds the
state for the first frame.

Backend now has zero `time::sleep`-driven polling loops: the syncer is
driven by Phase 0 + bootstrap + the event bus, prefetch is driven by
store changes, and the UI is driven by pushes. The only remaining
delays are throttling (`INTER_FOLDER_DELAY`, `INTER_REQUEST_DELAY`)
and reconnect backoff, which are not polls.

166 bridge lib tests, full workspace builds, UI tsc clean.

ui/src/hooks/useBridge.ts

@@ -1,10 +1,9 @@
-import { useState, useEffect, useCallback, useRef } from "react";
+import { useState, useEffect, useCallback } from "react";
 import { invoke } from "@tauri-apps/api/core";
 import { listen } from "@tauri-apps/api/event";
 import type { Config, BridgeStatus, BridgeStats } from "../types";
 
 const MAX_LOG_LINES = 500;
-const POLL_INTERVAL = 1000;
 
 export function useBridge() {
   const [config, setConfig] = useState<Config | null>(null);
@@ -18,7 +17,6 @@ export function useBridge() {
   const [bridgePassword, setBridgePassword] = useState<string | null>(null);
   const [logs, setLogs] = useState<string[]>([]);
   const [loading, setLoading] = useState(false);
-  const pollRef = useRef<ReturnType<typeof setInterval> | null>(null);
 
   const refresh = useCallback(() => {
     invoke<BridgeStatus>("get_status").then(setStatus);
@@ -31,9 +29,13 @@ export function useBridge() {
     invoke<string | null>("get_bridge_password").then(setBridgePassword);
     refresh();
 
-    pollRef.current = setInterval(refresh, POLL_INTERVAL);
+    // The bridge pushes `bridge://stats` and `bridge://status` whenever
+    // anything changes (mail count, ws state, start/stop). No setInterval.
+    const unlistenStats = listen<BridgeStats>("bridge://stats", (e) => setStats(e.payload));
+    const unlistenStatus = listen<BridgeStatus>("bridge://status", (e) => setStatus(e.payload));
     return () => {
-      if (pollRef.current) clearInterval(pollRef.current);
+      unlistenStats.then((fn) => fn());
+      unlistenStatus.then((fn) => fn());
     };
   }, [refresh]);