Make exporting a calendar work, and CalDAV publishing findable

Exporting any calendar whose name is not pure ASCII failed. str.isalnum() is
true for "ö", so the "safe" filename kept the umlaut and went into
Content-Disposition raw — an HTTP header must be ASCII, and a reverse proxy
drops the response. The client then had no JSON to read and showed its
"unknown error" fallback, which said nothing at all. attachments_router had
already learned this and documents it in a comment; the export endpoint never
got the same treatment. Now RFC 6266, exactly as that router does it.

downloadRequest discarded a non-JSON error body entirely. That body is the only
clue when the failure happens outside the app, so it is used now, with the
status appended when there is nothing to read.

The CalDAV publish button rendered as "..." — four buttons never fitted the
240px actions column, and the cell clips instead of overflowing because the
columns are drag-resizable. Nobody could tell there was a button there, which
is why publishing a calendar to Home Assistant looked impossible: the URL, the
copy button and both hints only appear once it is switched on. The label is now
just "CalDAV" with the full action as its tooltip, which also helps anyone who
already dragged that column narrower, since saved widths beat the default.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/js/api.js

@@ -86,8 +86,13 @@ async function downloadRequest(path, fallbackName) {
     return null;
   }
   if (!res.ok) {
-    const err = await res.json().catch(() => ({ detail: t('unknown_error') }));
-    const e = new Error(err.detail || `HTTP ${res.status}`);
+    // A non-JSON body means the failure happened outside the app — a reverse
+    // proxy rejecting the response, say. Showing "unknown error" there hides
+    // the one clue there is, so fall back to the body text and the status.
+    const raw = await res.text().catch(() => '');
+    let detail = '';
+    try { detail = JSON.parse(raw).detail || ''; } catch { detail = raw.trim().slice(0, 200); }
+    const e = new Error(detail || t('unknown_error') + ' (HTTP ' + res.status + ')');
     // Keep the status so callers can map it to a translated message instead of
     // showing the backend's German detail to someone running the app in Finnish.
     e.status = res.status;