feat: closed-set admin knobs are pickers, not free text

log_level, search_source, download_layout and the transcode hw/preset
were typed by hand, and every consumer carried its own guess at what a
typo meant. The value set is now a Literal in config.py: pydantic
rejects a bad env value at boot (any letter case accepted), sysconfig
refuses a bad override and derives the admin picker's options from the
same Literal, and the consumers read the value as-is.

The Configuration page renders a choice key as a segmented bar, or a
dropdown past six options, with translated option names. Its tabs now
follow the registry's order instead of a hand-kept copy that had
already dropped the explore group.

A single-choice FilterSelect now opens on the option in use: in the
browser it could land on the first row instead.

frontend/src/components/ui/FilterSelect.tsx

@@ -21,6 +21,9 @@ export interface FilterSelectOption {
 export interface FilterSelectProps {
   /** Visible group name shown as the trigger prefix (`Label: value`) and the accessible name. */
   label: string;
+  /** Show only the value on the trigger — for a form row whose own caption already names the field.
+   *  The label still names the control and its menu for assistive tech. */
+  hideLabel?: boolean;
   options: readonly FilterSelectOption[];
   selected: string[];
   onChange: (next: string[]) => void;
@@ -39,6 +42,7 @@ export default function FilterSelect({
   onChange,
   multiple = false,
   requireOne = true,
+  hideLabel = false,
   testId,
 }: FilterSelectProps) {
   const [open, setOpen] = useState(false);
@@ -47,6 +51,14 @@ export default function FilterSelect({
     onOpenChange: setOpen,
     mode: "menu",
     placement: "bottom-start",
+    // Single choice: opening lands on the option in use, not the first row. A multi-select toggle
+    // list has no one "current" row, so it keeps the default.
+    selectedIndex: multiple
+      ? null
+      : Math.max(
+          0,
+          options.findIndex((o) => o.value === selected[0]),
+        ),
   });
 
   const labelOf = (v: string) => options.find((o) => o.value === v)?.label ?? v;
@@ -88,7 +100,7 @@ export default function FilterSelect({
           open ? "border-accent" : "border-border"
         }`}
       >
-        <span className="text-muted">{label}:</span>
+        {!hideLabel && <span className="text-muted">{label}:</span>}
         <span className="text-fg">{summary}</span>
         <ChevronDown className="h-3.5 w-3.5 shrink-0 text-muted" aria-hidden="true" />
       </button>