Add in-app updater functionality

src/App.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
   import { onDestroy, onMount } from "svelte";
   import { open as openDialog } from "@tauri-apps/plugin-dialog";
+  import { check, type DownloadEvent, type Update } from "@tauri-apps/plugin-updater";
   import { AlertCircle, Check, FolderOpen, GitBranch, GitMerge, LoaderCircle } from "@lucide/svelte";
 
   import TitleBar from "./lib/TitleBar.svelte";
@@ -15,6 +16,7 @@
   import HistoryPanel from "./lib/components/HistoryPanel.svelte";
   import ResolveDialog from "./lib/components/ResolveDialog.svelte";
   import StatusPanel from "./lib/components/StatusPanel.svelte";
+  import UpdateToast from "./lib/components/UpdateToast.svelte";
 
   import {
     checkoutBranch,
@@ -71,6 +73,8 @@
     stripAuthPrefix,
   } from "./lib/credentials";
 
+  type UpdateToastState = "available" | "downloading" | "installed" | "error";
+
   // ── State ──────────────────────────────────────────────────────────────────
 
   let repoPath = "";
@@ -111,6 +115,16 @@
   let lastStatusFingerprint = "";
   const AUTO_REFRESH_INTERVAL = 4000;
   let autoRefreshTimer: ReturnType<typeof setInterval> | undefined;
+  let updateToastOpen = false;
+  let updateToastState: UpdateToastState = "available";
+  let pendingUpdate: Update | null = null;
+  let updateVersion = "";
+  let updateCurrentVersion = "";
+  let updateProgress = 0;
+  let updateError = "";
+  let updateCheckInFlight = false;
+  let updateDownloadTotal = 0;
+  let updateDownloadedBytes = 0;
 
   // ── Derived ────────────────────────────────────────────────────────────────
 
@@ -133,6 +147,7 @@
 
   onMount(() => {
     autoRefreshTimer = setInterval(() => { void autoRefreshTick(); }, AUTO_REFRESH_INTERVAL);
+    void checkForUpdates();
   });
 
   onDestroy(() => {
@@ -166,6 +181,78 @@
     if (autoRefreshEnabled) void autoRefreshTick();
   }
 
+  // ── Updates ────────────────────────────────────────────────────────────────
+
+  async function checkForUpdates() {
+    if (updateCheckInFlight || updateToastState === "downloading") return;
+    updateCheckInFlight = true;
+
+    try {
+      const update = await check();
+      if (!update) return;
+
+      if (pendingUpdate && pendingUpdate !== update) {
+        void pendingUpdate.close().catch(() => {});
+      }
+
+      pendingUpdate = update;
+      updateVersion = update.version;
+      updateCurrentVersion = update.currentVersion;
+      updateProgress = 0;
+      updateError = "";
+      updateToastState = "available";
+      updateToastOpen = true;
+    } catch {
+      // Update checks should be quiet when offline or when the endpoint is unavailable.
+    } finally {
+      updateCheckInFlight = false;
+    }
+  }
+
+  function updateDownloadProgress(event: DownloadEvent) {
+    if (event.event === "Started") {
+      updateProgress = 0;
+      updateDownloadTotal = event.data.contentLength ?? 0;
+      updateDownloadedBytes = 0;
+      return;
+    }
+
+    if (event.event === "Progress") {
+      updateDownloadedBytes += event.data.chunkLength;
+      updateProgress = updateDownloadTotal > 0
+        ? Math.min(99, Math.round((updateDownloadedBytes / updateDownloadTotal) * 100))
+        : 0;
+      return;
+    }
+
+    updateProgress = 100;
+  }
+
+  async function installPendingUpdate() {
+    if (!pendingUpdate || updateToastState === "downloading") return;
+
+    updateToastState = "downloading";
+    updateProgress = 0;
+    updateError = "";
+    updateToastOpen = true;
+    updateDownloadTotal = 0;
+    updateDownloadedBytes = 0;
+
+    try {
+      await pendingUpdate.downloadAndInstall(updateDownloadProgress);
+      updateProgress = 100;
+      updateToastState = "installed";
+    } catch (error) {
+      updateToastState = "error";
+      updateError = errorToMessage(error);
+    }
+  }
+
+  function dismissUpdateToast() {
+    if (updateToastState === "downloading") return;
+    updateToastOpen = false;
+  }
+
   // ── Utilities ──────────────────────────────────────────────────────────────
 
   function applyStatus(nextStatus: GitStatus) {
@@ -1009,6 +1096,19 @@
   </div>
 </main>
 
+{#if updateToastOpen}
+  <UpdateToast
+    state={updateToastState}
+    version={updateVersion}
+    currentVersion={updateCurrentVersion}
+    progress={updateProgress}
+    error={updateError}
+    onInstall={installPendingUpdate}
+    onLater={dismissUpdateToast}
+    onDismiss={dismissUpdateToast}
+  />
+{/if}
+
 <!-- Compare diff dialog -->
 {#if compareDialogOpen && comparison}
   <CompareDialog