fix(hub): the show detail modal must not move when the season does

The previous pass fixed the synopsis and the cast, and the dialog still
jumped: the episode count moves things a fixed-height synopsis cannot reach.

- The body scrolled as a whole, so a thirteen-episode season pushed the modal
  to its max-height where a six-episode one had not. `.video-overlay` centres
  its child, so the taller modal also *started higher up the screen* — title
  bar, close button and all. `.video-detail-steady` (a multi-season show only)
  gives the modal a height rather than a max-height, makes the body a flex
  column, and hands the leftover to the episode list as the one scrolling
  part. A constant-height box is centred in the same place every time, so both
  halves settle at once.
- A scrolling season draws a scrollbar where a non-scrolling one draws none,
  which is a scrollbar's width of content and re-wrapped the file path above
  it, shifting everything below by a line. `scrollbar-gutter: stable`.
- The season panel was clipped by the modal's own `overflow: hidden` whenever
  the seasons outran the room under the picker — at a 740px viewport it wanted
  320px and had 288, and the rest sat where no scroll could reach it. It is
  `position: fixed` now, placed by `placeSeasonPanel()`, which takes the
  trigger's rect and the window height, picks whichever side has more room,
  and caps the panel to it.

Scoped to multi-season shows throughout: a movie has no season to switch to
and a fixed height would buy it nothing but empty space.

test_video_detail_measured.py now builds each block inside a real
`.video-overlay`, since the centring is half the defect, and asserts the modal
top and height as well as the picker's offset — for a long and a short
synopsis and for a six- and a twenty-four-episode season.
test_season_panel_placement.py runs placeSeasonPanel() in node over a rect and
a window height. Two guards are declarations rather than rectangles and say so
in their docstrings: headless Chrome gives the probe zero-width overlay
scrollbars, so the gutter cannot be measured there.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014UtzVrzM7e2tG9fSpkR9ML

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