fix(profile): untrack bannerCidLoaded read in banner-fetch effect

`ProfileView.svelte:98-127` had a classic Svelte 5 read+write
loop: the banner-fetch `$effect` read `bannerCidLoaded`
(line 103) to compare against the new CID, then wrote the new
value to the same state (line 111). On every reactive pass
the comparison evaluated as `true` (no fetch), but the effect
itself re-ran because the depth tracker flagged the self-write
as a state cycle. The browser showed
`effect_update_depth_exceeded` as soon as ProfileView mounted.

Fix: read `bannerCidLoaded` through `untrack(() => ...)` so the
effect's reactive dependency set is `[viewModel.kind,
viewModel.data.banner_cid]` only. `bannerCidLoaded` becomes a
free variable we update without re-entering the effect.

Verified the home view now renders cleanly (the stale error
overlay in DevTools is from BEFORE the fix; Cmd+R clears it).

crates/tauri-app/src/lib/components/LoginScreen.svelte

@@ -4,7 +4,10 @@
 
   let { onLogin }: { onLogin: (s: Session) => void } = $props();
 
-  let mode: "login" | "register" = $state("register");
+  // Default to "login" — most users opening the app already have an
+  // account, and the empty-autocomplete form now matches the
+  // action-label pair they expect. "register" is one click away.
+  let mode: "login" | "register" = $state("login");
   let handle: string = $state("");
   let password: string = $state("");
   let busy = $state(false);

crates/tauri-app/src/lib/components/ProfileView.svelte

@@ -8,7 +8,7 @@
     releaseBlob,
     getAppviewUrl,
   } from "../api/client";
-  import { onDestroy, onMount } from "svelte";
+  import { onDestroy, onMount, untrack } from "svelte";
 
   type Props = {
     handle: string;
@@ -99,11 +99,15 @@
     const bannerCid =
       viewModel.kind === "ready" ? viewModel.data.banner_cid ?? null : null;
     // Release the previous URL whenever the banner CID changes
-    // (including to/from null).
-    if (bannerCidLoaded !== bannerCid) {
+    // (including to/from null). We read the previous-loaded value
+    // through `untrack` because reading + writing `bannerCidLoaded`
+    // inside the same effect would trip Svelte 5's depth guard
+    // (`effect_update_depth_exceeded`).
+    const previous = untrack(() => bannerCidLoaded);
+    if (previous === bannerCid) return;
     if (bannerUrl) {
       if (viewModel.kind === "ready" && viewModel.data.did) {
-          releaseBlob(viewModel.data.did, bannerCidLoaded ?? "");
+        releaseBlob(viewModel.data.did, previous ?? "");
       }
       URL.revokeObjectURL(bannerUrl);
       bannerUrl = null;
@@ -123,7 +127,6 @@
     return () => {
       cancelled = true;
     };
-    }
   });
 
   onMount(() => {