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.

CLAUDE.md

@@ -102,6 +102,7 @@ CQRS, a repository/DI framework, a router library, a generic CRUD-router factory
 | FE static label / button-like link   | `Badge` (flat, no edge) · `LinkButton` / `IconLink`                                                                                                                                               | a glass pill that reads as a chip; an `<a>` hand-painted like a button                                                                                  |
 | FE loading/empty/error               | `QueryState` / `StateMessage`                                                                                                                                                                     | ad-hoc conditionals                                                                                                                                     |
 | FE failed write                      | `meta.failTitle`, else `notifyActionError(err, title)`                                                                                                                                            | a bare `catch {}` or `notify()` with no error attached                                                                                                  |
+| FE player on a native media element  | `lib/mediaSession` (`useMediaSession`) + `lib/mediaKeys` + `lib/resume` (`useResume`), the element held as STATE                                                                                  | a private `navigator.mediaSession` / `keydown` / position-saving effect per player                                                                      |
 | FE diagnostics                       | `lib/log.ts` (`log.info("event.code", msg, attrs)`); a filter change → `lib/filterLog.ts`, a player → `lib/mediaLog.ts`                                                                           | `console.*` (lint-enforced); a value only a higher level may carry, logged unguarded                                                                    |
 | FE state that MIRRORS a prop/query   | derive it, or hold a draft (`draft ?? server`)                                                                                                                                                    | a copy an effect re-syncs — that paints the stale value first                                                                                           |
 | FE state reset on a key change       | adjust it DURING render against a remembered key                                                                                                                                                  | `useEffect(() => setX(init), [key])`                                                                                                                    |