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/components/PaneCP.js

@@ -1,5 +1,6 @@
-import { h, ref, watch } from 'vue';
-import { fetchScoreText, putScoreText, fetchAudioWidget, URLS } from '../api.js';
+import { h, ref, watch, onMounted } from 'vue';
+import { fetchAstLog, fetchScoreText, putScoreText, fetchAudioWidget, URLS } from '../api.js';
+import { parseAstLog, buildModel } from '../ast-parser.js';
 import { patchScore } from '../exporter.js';
 import { StatusPoller } from './StatusPoller.js';
 
@@ -44,8 +45,10 @@ function shortView(node) {
 }
 
 export const PaneCP = {
-    props: ['store', 'onImportClick', 'onFocusFO'],
+    props: ['store', 'importOnLoad', 'onFocusFO'],
     setup(props) {
+        const importing = ref(false);
+        const importError = ref('');
         const exporting = ref(false);
         const exportError = ref('');
         const audioWidgetHtml = ref('');
@@ -61,18 +64,33 @@ export const PaneCP = {
             },
         );
 
+        async function doImport() {
+            if (props.store.isDirty) return;
+            importError.value = '';
+            importing.value = true;
+            try {
+                const text = await fetchAstLog();
+                props.store.scoreModel = buildModel(parseAstLog(text));
+                props.store.exportLog = [];
+            } catch (e) {
+                importError.value = e.message;
+            } finally {
+                importing.value = false;
+            }
+        }
+
         async function doExport() {
             exportError.value = '';
             exporting.value = true;
             try {
-                const raw = await fetchScoreText(props.store.credentials);
+                const raw = await fetchScoreText();
                 props.store.rawScoreText = raw;
                 const model = props.store.scoreModel;
                 const { text: patched, log } = patchScore(
                     raw, model.instruments, model.bars, model.info, model.articles ?? [],
                 );
                 props.store.exportLog = log;
-                await putScoreText(patched, props.store.credentials);
+                await putScoreText(patched);
                 props.store.synthesisStatus = { frozen: false, currently_rendered_notes: 0, notes_in_total: 0 };
             } catch (e) {
                 exportError.value = e.message;
@@ -81,6 +99,8 @@ export const PaneCP = {
             }
         }
 
+        onMounted(() => { if (props.importOnLoad) doImport(); });
+
         return () => {
             const store = props.store;
             const model = store.scoreModel;
@@ -97,10 +117,10 @@ export const PaneCP = {
                 h('div', { class: 'se-cp-header' }, [
                     h('button', {
                         class: 'se-btn',
-                        disabled: store.isDirty,
+                        disabled: store.isDirty || importing.value,
                         title: store.isDirty ? 'Save or discard edits before re-importing' : 'Import from server',
-                        onClick: props.onImportClick,
-                    }, '→ Import'),
+                        onClick: doImport,
+                    }, importing.value ? 'Importing…' : '→ Import'),
                     h('span', { class: 'se-cp-title' },
                         model ? (model.info?.title ?? 'Untitled score') : 'No score loaded'),
                     model ? h('button', {
@@ -135,7 +155,8 @@ export const PaneCP = {
                     })
                 ) : null,
 
-                // Export error
+                // Import / export errors
+                importError.value ? h('div', { class: 'se-error' }, importError.value) : null,
                 exportError.value ? h('div', { class: 'se-error' }, exportError.value) : null,
 
                 // Export log (shown after export, cleared on next import)