fix(settings): stop clobbering WebDAV edits and save without a successful test

Two coupled issues in workspace settings:

1. The credentials-loading effect re-ran whenever ws.webdav_url changed,
   so any config mutation (e.g. changing sync interval) would trigger a
   re-load of the stored username/password, overwriting whatever the
   user was typing into those fields. Gate with a one-shot credsLoaded
   flag.

2. Save would persist whatever was in the URL input even if the user
   had never tested it — a typo'd host silently pointed the workspace
   at a dead server. Now saveWebdav auto-runs the connection test and
   bails if it fails; any edit to the three inputs clears the "ok"
   status via markDirty() so the next Save is forced to re-verify.

Also replaces the ASCII "Failed -- Retry" with an em dash.

apps/tauri/src/lib/screens/SettingsScreen.svelte

@@ -15,14 +15,19 @@
   let webdavUser = $state("");
   let webdavPass = $state("");
   let testStatus = $state<"idle" | "testing" | "ok" | "fail">("idle");
+  let credsLoaded = $state(false);
 
   let renaming = $state(false);
   let renameValue = $state("");
   let showKebab = $state(false);
   let confirmRename = $state(false);
 
+  // Load stored credentials exactly once for this workspace. Previously this
+  // ran on every `ws.webdav_url` change, which silently clobbered in-progress
+  // user edits whenever any other setting updated the config.
   $effect(() => {
-    if (!ws?.webdav_url) return;
+    if (credsLoaded || !ws?.webdav_url) return;
+    credsLoaded = true;
     webdavUrl = ws.webdav_url;
     try {
       const domain = new URL(ws.webdav_url).hostname;
@@ -35,6 +40,12 @@
     } catch {}
   });
 
+  // Any edit invalidates a prior test so users can't Save a config they
+  // haven't validated since changing it.
+  function markDirty() {
+    if (testStatus !== "idle") testStatus = "idle";
+  }
+
   async function testConnection() {
     testStatus = "testing";
     try {
@@ -51,6 +62,12 @@
 
   async function saveWebdav() {
     if (!webdavUrl.trim()) return;
+    // Require a successful test so a typo'd URL can't silently point the
+    // workspace at a dead server.
+    if (testStatus !== "ok") {
+      await testConnection();
+      if (testStatus !== "ok") return;
+    }
     await invoke("set_webdav_config", {
       workspaceId,
       webdavUrl: webdavUrl.trim(),
@@ -172,6 +189,7 @@
         <input
           type="url"
           bind:value={webdavUrl}
+          oninput={markDirty}
           placeholder="https://dav.example.com/tasks/"
           class="mb-3 w-full rounded-lg border border-border-light bg-transparent px-3 py-2 text-sm outline-none focus:border-primary dark:border-border-dark"
         />
@@ -180,6 +198,7 @@
         <input
           type="text"
           bind:value={webdavUser}
+          oninput={markDirty}
           class="mb-3 w-full rounded-lg border border-border-light bg-transparent px-3 py-2 text-sm outline-none focus:border-primary dark:border-border-dark"
         />
 
@@ -187,6 +206,7 @@
         <input
           type="password"
           bind:value={webdavPass}
+          oninput={markDirty}
           class="mb-4 w-full rounded-lg border border-border-light bg-transparent px-3 py-2 text-sm outline-none focus:border-primary dark:border-border-dark"
         />
 
@@ -196,7 +216,7 @@
             disabled={!webdavUrl.trim()}
             class="rounded-lg border border-border-light px-4 py-2 text-sm font-medium hover:bg-black/5 disabled:opacity-40 dark:border-border-dark dark:hover:bg-white/10"
           >
-            {testStatus === "testing" ? "Testing..." : testStatus === "ok" ? "Connected" : testStatus === "fail" ? "Failed -- Retry" : "Test Connection"}
+            {testStatus === "testing" ? "Testing…" : testStatus === "ok" ? "Connected" : testStatus === "fail" ? "Failed — Retry" : "Test Connection"}
           </button>
           <button
             onclick={saveWebdav}