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.

README.md

@@ -23,8 +23,9 @@ blocker and SponsorBlock keep working.
 - **Playlists with two-way YouTube sync** — build them locally, keep them in sync in both directions.
 - **In-app player** with resume, plus keyboard/scroll controls.
 - **Download Center** — save videos to the server with yt-dlp in a Plex-friendly layout (format
-  presets, per-user storage quota), trim / crop / split & join them in a built-in editor, then save
-  to your device, share with another user, or hand out a public watch link.
+  presets, per-user storage quota), trim / crop / split & join them in a built-in editor, play them
+  where you left off, then save to your device, share with another user, or hand out a public watch
+  link.
 - **Plex library** *(optional)* — point Siftlode at your own Plex server to browse and play its
   movies and shows alongside your feed, with watch state kept in sync.
 - **Multi-user** with per-user private state, a shared catalog, and a fair daily API-quota guard.