Drop custom auth dialog; rely on browser Basic Auth handling

Remove ImportDialog and all manual credential management. The browser
handles 401 challenges natively for same-origin fetch requests — it
shows its own credential prompt, retries, and caches the result for
the session. No JS-level auth code is needed.

- api.js: remove authHeader() and all credentials parameters
- store.js: remove credentials field
- ImportDialog.js: deleted
- AppShell.js: remove showImport/openImport/onMounted dialog machinery;
  pass importOnLoad through to PaneCP
- PaneCP.js: inline doImport() (was in ImportDialog); auto-triggers on
  mount when importOnLoad is true; shows inline import error on failure
- StatusPoller.js: remove credentials arg from fetchStatus()
- style.css: remove .se-import-dialog and .se-overlay blocks

static/api.js

@@ -2,45 +2,30 @@ const U = window.NEUSICIAN_URLS;
 
 export const URLS = U;
 
-function authHeader(credentials) {
-    if (!credentials) return {};
-    const b64 = btoa(`${credentials.username}:${credentials.password}`);
-    return { Authorization: `Basic ${b64}` };
-}
-
-export async function fetchAstLog(credentials) {
-    const res = await fetch(U.astlog, {
-        headers: authHeader(credentials),
-    });
+export async function fetchAstLog() {
+    const res = await fetch(U.astlog);
     if (!res.ok) throw new Error(`${res.status} ${res.statusText}`);
     return res.text();
 }
 
-export async function fetchScoreText(credentials) {
-    const res = await fetch(U.score, {
-        headers: authHeader(credentials),
-    });
+export async function fetchScoreText() {
+    const res = await fetch(U.score);
     if (!res.ok) throw new Error(`${res.status} ${res.statusText}`);
     return res.text();
 }
 
-export async function putScoreText(text, credentials) {
+export async function putScoreText(text) {
     const res = await fetch(U.score, {
         method: 'PUT',
-        headers: {
-            ...authHeader(credentials),
-            'Content-Type': 'text/yaml',
-        },
+        headers: { 'Content-Type': 'text/yaml' },
         body: text,
     });
     if (!res.ok) throw new Error(`${res.status} ${res.statusText}`);
     return res;
 }
 
-export async function fetchStatus(credentials) {
-    const res = await fetch(U.status, {
-        headers: authHeader(credentials),
-    });
+export async function fetchStatus() {
+    const res = await fetch(U.status);
     if (!res.ok) throw new Error(`${res.status} ${res.statusText}`);
     return res.json();
 }