feat(settings): Nextcloud server-URL hint + edit-mode test uses stored password

Provider-aware Server URL placeholder/hint (Nextcloud/ownCloud: enter just the
server URL); Test in edit mode passes the target id so the backend reuses the
stored credential.

src/lib/views/Settings.svelte

@@ -345,6 +345,10 @@
   let form = $state<SyncTargetConfig>(blankForm("webdav"));
   // When set, the form is editing an existing target rather than adding one.
   let editingId = $state<string | null>(null);
+  // Nextcloud/ownCloud only need the server URL — the app builds the DAV path.
+  let davAutoPath = $derived(
+    form.provider_hint === "nextcloud" || form.provider_hint === "owncloud",
+  );
 
   function blankForm(kind: SyncKind): SyncTargetConfig {
     return {
@@ -370,7 +374,9 @@
     testResult = null;
   }
   async function testConnection() {
-    const r = await settings.test(form);
+    // In edit mode, pass the id so the backend reuses the stored password
+    // (unless a new one was typed) while still testing the edited URL/fields.
+    const r = await settings.test(editingId ? { ...form, id: editingId } : form);
     testResult = { ok: r.ok, message: r.message };
   }
   // Load an existing target into the form for editing. The secret is never
@@ -786,9 +792,17 @@
             <label class="wide"
               >Server URL<input
                 bind:value={form.base_url}
-                placeholder="https://cloud.example.com/remote.php/dav/files/me/"
-              /></label
-            >
+                placeholder={davAutoPath
+                  ? "https://cloud.example.com"
+                  : "https://host/remote.php/dav/… (or /seafdav, /dav)"}
+              />
+              {#if davAutoPath}
+                <small class="muted"
+                  >Just your server URL — WhispAssist adds
+                  <code>/remote.php/dav/files/&lt;username&gt;/</code> automatically.</small
+                >
+              {/if}
+            </label>
             <label>Remote folder<input bind:value={form.remote_base_path} /></label>
             <label>Username<input bind:value={form.username} /></label>
             <label