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.

backend/app/plex/stream.py

@@ -404,7 +404,7 @@ def _select_encode(db: DbSession, item: PlexItem, transcode: bool, src: Path) ->
     source that has an OpenCL runtime, while a 10-bit SDR source drops to software."""
     if not transcode:
         return (False, False)
-    mode = (sysconfig.get_str(db, "plex_transcode_hw") or "auto").lower()
+    mode = sysconfig.get_str(db, "plex_transcode_hw")
     if mode == "off" or not _render_node_present():
         return (False, False)
     if mode == "on":
@@ -664,7 +664,7 @@ def start_session(
         # remux ignore preset/crf/max_height, and remux must not touch the DB for them).
         preset, crf, max_height = "veryfast", 20, 0
         if transcode and not hw:
-            preset = sysconfig.get_str(db, "plex_transcode_preset") or "veryfast"
+            preset = sysconfig.get_str(db, "plex_transcode_preset")
             crf = sysconfig.get_int(db, "plex_transcode_crf")
             max_height = sysconfig.get_int(db, "plex_transcode_max_height")
         # Nice the SOFTWARE transcode child (not the HW path, and not remux) so a CPU-bound re-encode