fix(tauri-app): use absolute AppView URL for /api/profile fetch

The Tauri webview's origin is the Vite dev server (port 1430), not
the AppView (port 2584). A relative `fetch('/api/profile/…')`
resolves against Vite, which has no proxy configured, so the
request lands on Vite's 404 HTML page and `response.json()` then
throws `SyntaxError: The string did not match the expected
pattern.` The error surfaced as `err: SyntaxError…` under the
banner of the redesigned ProfileView.

Fix: expose the AppView base URL the Tauri shell was started with
as a sync `get_api_urls` Tauri command. The Rust side reads the
URL from `MAARCADETWEET_APPVIEW_URL` (default
`http://127.0.0.1:2584`) at startup and stores it on `AppState`
so the command doesn't need to re-read the env. The frontend
exposes a cached `getAppviewUrl()` helper; ProfileView's
`load()` uses it to build an absolute fetch URL.

The relative-path bug also affected the previous UserProfileView,
but it never errored loudly enough for the user to notice — the
new X-style layout made the err block visible.

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

@@ -6,6 +6,7 @@
     pickAndUploadImage,
     fetchBlob,
     releaseBlob,
+    getAppviewUrl,
   } from "../api/client";
   import { onDestroy, onMount } from "svelte";
 
@@ -74,7 +75,12 @@
   async function load() {
     viewModel = { kind: "loading" };
     try {
-      const r = await fetch(`/api/profile/${encodeURIComponent(handle)}`);
+      // Absolute URL because the Tauri webview's origin is the Vite
+      // dev server (port 1430), not the AppView (port 2584) — a
+      // relative `/api/profile/…` would resolve against Vite, hit a
+      // 404 HTML page, and `r.json()` would throw `SyntaxError`.
+      const base = await getAppviewUrl();
+      const r = await fetch(`${base}/api/profile/${encodeURIComponent(handle)}`);
       if (!r.ok) {
         viewModel = {
           kind: "error",