feat(player): the OS media controls, the basic keys and resume for the library and shared-link players

Plex, library and /watch tell the operating system what plays (title, line, picture) and take its
play / pause / skip / seek through one hook (lib/mediaSession); the Plex player drives it with its own
seek and clock, so the lock screen shows the film's position, not the stream's. The YouTube player is
left alone: its iframe already publishes all of it, and the OS routes to the frame that plays.

The library and /watch players get the basic keys (lib/mediaKeys, the keymap's mediaPlayer rows) and
open where the viewer stopped (lib/resume): the library keeps the point on the server, per user and
library item (library_states, POST /api/downloads/{id}/progress, listed back on both listings), the
shared link in this browser. A started library row continues, its menu can start it over, and its
picture carries the cards' progress bar. The resume rule (too early / near the end = from the start)
is one function shared with the YouTube route.

The library dialog opens focused on its panel (Modal initialFocus "player"): on the ✕, Space closed
the film. While the media element itself has the focus, Space and the arrows stay its own controls'
— the browser acts on them whatever a listener does, and one Space played and paused.

docs/DESIGN.md

@@ -528,7 +528,9 @@ of a full-width panel and nothing on the right.
   **Restart**, the video card's own pair and words (`MediaResumeButtons`, one component for every card and
   Home's hero). A poster stacks the pair; a landscape picture sets it side by side, icon-only on a small one.
   On touch a tap continues and the card's "…" has "Play from the beginning" under its Play row. The restart
-  applies to the item the player opened on (`fromStart`); every item it moves on to resumes.
+  applies to the item the player opened on (`fromStart`); every item it moves on to resumes. A library
+  row is not a card and keeps its one play button: started, that button continues, its "…" has Continue +
+  "Play from the beginning", and its picture carries the cards' progress bar.
 - **The YouTube modal:** its chrome uses the `Button` rungs (44 on touch), the side arrows stay inside the
   screen, and the description opens from an ⓘ button, because a hover popover never opens on a tap (measured
   live). [WCAG 1.4.13]
@@ -541,7 +543,9 @@ of a full-width panel and nothing on the right.
   fields (the issue composer's Edit / Preview switch); one that loads after opening moves the focus
   there when it arrives, as long as the focus still rests on the ✕;
   `action` — a confirm, where the least destructive button takes `autoFocus`; `close` — anything to read,
-  browse or manage. [WAI-ARIA APG]
+  browse or manage; `player` — a dialog whose body is a player (the library's): the focus rests on the
+  panel itself, so Space and the arrows reach the player's keys and no button takes them — Space on a
+  focused ✕ closed the film. [WAI-ARIA APG]
 - **The header and the footer are fixed; only the body scrolls,** with the B9 fade at each edge, so the
   primary action stays on screen when the on-screen keyboard takes half of it (the viewport's
   `interactive-widget=resizes-content` makes the keyboard shrink the layout, where Chrome would otherwise
@@ -706,3 +710,4 @@ of a full-width panel and nothing on the right.
 | 2026-10-02 | E31 Plex detail poster and crew                  | The movie and show poster keeps its 2:3 and grows a rung in a wide hero (w-56 at a 64rem panel); the crew shows its first 6 names and "Show N more" (a list of 8 or fewer shows whole)                                                                                                                                 | Poster stretched to the row; a "More" on the keywords                                                                                                                                                  | The row is as tall as the crew column, so the poster drew ~175×590 and cropped; the crew, not the keywords, set the hero's height                                                             |
 | 2026-10-04 | E32 `field` focus skips a mode switch | A form opens on its first field; a control above it that is not a field (the composer's Edit / Preview) is passed over by marking the field `data-initial-focus` | The first text field everywhere | That moved the focus in the download dialog off its profile choice and in the upload dialog off its drop zone; the mark changes only the form that needs it |
 | 2026-10-04 | E35 Lint for z-levels and edged labels | Both rules stay; the two eslint selectors (`Z_RAW`, `EDGED_LABEL`) that refused them go | Keep the selectors | 0 hits since the E28 sweep that fixed the sites: a check that never fires costs a probe test and a snapshot each, and the UI review holds both rules |
+| 2026-10-06 | `player` focus | A dialog that is a player opens focused on its panel (the YouTube player's rule, now `Modal`'s too); on the media element itself, Space and the arrows stay the element's own | `close`, or the media element | On the ✕ Space closed the player; on the element the browser's own controls acted on the same key as the player's (one Space played and paused, measured in Chrome) |