Add sidecar update check on startup

On launch, after confirming the sidecar is installed, the app now
checks for a newer sidecar version via the Gitea API. If an update
is available, shows a prompt with "Update Now" or "Skip":

- Update Now: shows the SidecarSetup download screen
- Skip: launches the existing sidecar version

The update check is non-blocking -- if it fails (no internet, API
error), the app silently proceeds with the current version.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/App.svelte

@@ -9,11 +9,12 @@
   import { backendStore } from "$lib/stores/backend";
   import { configStore } from "$lib/stores/config";
 
-  type SidecarState = "checking" | "needs_setup" | "starting" | "connected";
+  type SidecarState = "checking" | "needs_setup" | "update_available" | "starting" | "connected";
 
   let showSettings = $state(false);
   let sidecarState = $state<SidecarState>("checking");
   let debugLog = $state("");
+  let availableUpdate = $state("");
 
   let obsDisplayUrl = $derived(backendStore.obsUrl);
   let syncDisplayUrl = $derived(backendStore.syncUrl);
@@ -53,6 +54,20 @@
         return;
       }
 
+      // Check for sidecar updates before launching
+      try {
+        log("Checking for sidecar updates...");
+        const update = await invoke<string | null>("check_sidecar_update");
+        if (update) {
+          log(`Sidecar update available: ${update}`);
+          availableUpdate = update;
+          sidecarState = "update_available";
+          return;
+        }
+      } catch (err) {
+        log(`Update check failed (non-fatal): ${err}`);
+      }
+
       await launchSidecar();
     } catch (err) {
       // Not running in Tauri (browser dev mode) - skip sidecar check
@@ -118,6 +133,26 @@
 {:else if sidecarState === "needs_setup"}
   <SidecarSetup onComplete={onSidecarReady} />
 
+{:else if sidecarState === "update_available"}
+  <div class="connecting-overlay" style="background:#1e1e1e;color:#e0e0e0;display:flex;align-items:center;justify-content:center;height:100%;width:100%;">
+    <div class="connecting-content" style="text-align:center;max-width:400px;">
+      <h2 style="font-size:20px;margin:0 0 12px;">Sidecar Update Available</h2>
+      <p style="color:#a0a0a0;font-size:14px;margin:0 0 20px;">
+        A new version of the transcription engine is available ({availableUpdate}).
+      </p>
+      <div style="display:flex;gap:10px;justify-content:center;">
+        <button
+          style="padding:8px 20px;border:1px solid #555;border-radius:6px;background:transparent;color:#e0e0e0;cursor:pointer;"
+          onclick={() => launchSidecar()}
+        >Skip</button>
+        <button
+          style="padding:8px 20px;border:none;border-radius:6px;background:#4CAF50;color:white;cursor:pointer;font-weight:500;"
+          onclick={() => { sidecarState = "needs_setup"; }}
+        >Update Now</button>
+      </div>
+    </div>
+  </div>
+
 {:else if !isConnected}
   <div class="connecting-overlay" style="background:#1e1e1e;color:#e0e0e0;display:flex;align-items:center;justify-content:center;height:100%;width:100%;">
     <div class="connecting-content" style="text-align:center;">