contact messages: browser-side PoW solving in the contact panel

The bridge routes {type: pow} frames to the panel. contact-main solves
each fresh challenge in the background with the backend-served easypow
solver module (imported by URL from a variable so Rollup leaves it
dynamic), aborting stale solves on replacement challenges. Sending
awaits the current solution and burns it with the message; a
'required' rejection re-solves and retries the message once.

Composer and ContactPanel need no changes: uploads stay ungated, and
the send path just awaits the in-flight solve.

frontend/src/contact-main.js

@@ -31,6 +31,55 @@ function markSeen() {
   refreshBadges()
 }
 
+// PoW (easypow, docs/messages.md): the server issues a fresh single-use
+// challenge with every registration/resync and after every accepted
+// message; one solution burns one message. The solver module is served
+// by the backend from the easypow package, so both sides always match.
+const SOLVER_URL = '/_contact/easypow.js' // backend-served; kept in a variable so Rollup leaves the import dynamic
+let bridge = null
+let token = ''
+let solverPromise = null
+let powSolution = null // Promise<string> solving the current challenge
+let powAborter = null
+let unsent = null // last message text, retried once if its solution was rejected
+
+// The solution for the currently outstanding challenge; rides out a
+// challenge replacement (reconnect) by continuing with the new solve.
+async function currentSolution() {
+  for (;;) {
+    const p = powSolution
+    if (!p) return ''
+    try {
+      return await p
+    } catch {
+      if (p === powSolution) return '' // failed without a replacement
+    }
+  }
+}
+
+async function sendMessage(md) {
+  unsent = md
+  const pow = await currentSolution()
+  if (pow) bridge.send({ token, msg: md, pow })
+}
+
+async function onPow(msg) {
+  powAborter?.abort() // superseded challenge: stop the stale solve
+  const ac = (powAborter = new AbortController())
+  solverPromise ??= import(/* @vite-ignore */ SOLVER_URL)
+  const { solve } = await solverPromise
+  powSolution = solve(msg.challenge, ac.signal)
+  powSolution.catch(() => {}) // aborts are expected
+  // A rejected send (stale or already burned solution) is retried once
+  // the fresh challenge is solved; a second rejection drops it.
+  if (msg.required && unsent) {
+    const md = unsent
+    unsent = null
+    const pow = await currentSolution()
+    if (pow) bridge.send({ token, msg: md, pow })
+  }
+}
+
 // A "new reply" dot on every contact trigger button while the panel is
 // closed and the newest admin message is younger than the last look.
 // Called on thread pushes and (from pagerite.js) after navigations,
@@ -49,13 +98,14 @@ function onThread(thread) {
   else refreshBadges()
 }
 
-export function open(bridge) {
+export function open(br) {
+  bridge = br
   if (!app) {
-    const token = ensureToken()
+    token = ensureToken()
     app = createApp(ContactPanel, {
       state,
       token,
-      onSend: (md) => bridge.send({ token, msg: md }),
+      onSend: (md) => sendMessage(md),
       onClose: () => {
         state.open = false
         refreshBadges()
@@ -67,7 +117,7 @@ export function open(bridge) {
     app.mount(el)
     // Registration doubles as the resync: the server pushes the thread
     // state back, and pagerite.js re-registers on every socket reconnect.
-    bridge.register(token, onThread)
+    bridge.register(token, onThread, onPow)
   }
   state.open = true
   markSeen()

frontend/src/pagerite.js

@@ -643,6 +643,7 @@ import { reconnectPolicy, socketSlot, watchConnecting } from "./reconnect";
         return;
       }
       if (msg?.type === "thread") contactListener(msg);
+      else if (msg?.type === "pow") contactPowListener?.(msg);
       else if (msg?.type === "contact-error") console.warn("contact:", msg.detail);
     };
     ws.onclose = () => {
@@ -893,10 +894,12 @@ import { reconnectPolicy, socketSlot, watchConnecting } from "./reconnect";
   let contactMod = null;
   let contactToken = "";
   let contactListener = null;
+  let contactPowListener = null;
   const contactBridge = {
-    register(token, onThread) {
+    register(token, onThread, onPow) {
       contactToken = token;
       contactListener = onThread;
+      contactPowListener = onPow;
       contactBridge.send({ token });
     },
     send(msg) {