docs: split feature details into docs/

One page per feature group (sessions, keyboard, launcher menu, dashboard, SteamVR debugger, Firefox, Jellyfin, UI patches, changes outside Nix) and the full options table in docs/options.md.

docs/jellyfin.md

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+# Jellyfin hardware decoding
+
+`jellyfin.hardwareDecoding.enable`, for the Flathub
+[Jellyfin Desktop](https://github.com/jellyfin/jellyfin-desktop) Flatpak
+(`org.jellyfin.JellyfinDesktop`), which plays video with libmpv. Options:
+[options.md#jellyfin](options.md#jellyfin).
+
+**Problem:** video is decoded in software (1080p H.264: ~40 % CPU). The
+Frame's hardware decoder is a V4L2 memory-to-memory device (`qcom-iris`,
+`/dev/video*`), which
+
+- the Flatpak can't open: its `devices=dri` covers only the GPU, and Flatpak
+  has nothing between that and `devices=all`;
+- mpv never tries: Jellyfin hard-sets `hwdec=auto-copy`, and mpv's `auto`
+  probing leaves out V4L2 M2M on purpose (its quality varies by SoC).
+  Jellyfin has no way to pass mpv options.
+
+**What it does:** device access (`devices=all`) and an `LD_PRELOAD` shim
+that rewrites an `hwdec` value starting with `auto` (set through libmpv's
+`mpv_set_*` functions) to `$SFN_MPV_HWDEC` (the `hwdec` option, default
+`v4l2m2m-copy,auto-copy`); explicit values such as `no` stay. mpv tries the
+listed decoders in order and falls back to software decoding per stream.
+With the default, 1080p H.264 plays through `v4l2m2m-copy` at ~15-20 % CPU.
+Changes take effect at the next start of Jellyfin.
+
+Installing the Flatpak is up to you, e.g.
+`flatpak install --user flathub org.jellyfin.JellyfinDesktop`, or with
+nix-flatpak:
+
+```nix
+services.flatpak.packages = [ "org.jellyfin.JellyfinDesktop" ];
+```
+
+**Caveats:**
+
+- `devices=all` gives the app all of `/dev` (cameras, input devices, ...),
+  not just the decoder.
+- V4L2 M2M decoding quality varies with drivers and codecs. Tested: 8-bit
+  H.264; 10-bit HEVC is untested. mpv falls back to software only when the
+  decoder fails; for streams that decode with artifacts, disable the option
+  (or set `hwdec = "auto-copy"`, Jellyfin's own value).
+
+## How it works
+
+The shim (a few libmpv wrappers, only libc) is preloaded from the Nix store;
+only its store path is exposed (read-only) to the sandbox. It would work for
+any libmpv app that sets `hwdec=auto*`, but only Jellyfin Desktop is set up
+here.
+
+Nothing is written to Flatpak's overrides: a desktop entry shadowing the
+Flatpak's (`~/.local/share/applications/org.jellyfin.JellyfinDesktop.desktop`,
+same ID, so the KDE menu and the "+" menu start it) passes them as
+`flatpak run` options, so they apply to launches from that entry and are
+gone with it. From a terminal:
+
+```sh
+flatpak run --branch=stable --arch=aarch64 --command=jellyfin-desktop \
+  --device=all --filesystem=<shim>:ro \
+  --env=LD_PRELOAD=<shim>/lib/mpv-hwdec-shim.so \
+  --env=SFN_MPV_HWDEC=v4l2m2m-copy,auto-copy org.jellyfin.JellyfinDesktop
+```
+
+The exact line with the store path: `grep ^Exec=
+~/.local/share/applications/org.jellyfin.JellyfinDesktop.desktop`, or the
+read-only option `steamFrame.jellyfin.hardwareDecoding.command`. In its
+output: `mpv-hwdec-shim: hwdec "auto-copy" -> "v4l2m2m-copy,auto-copy"`,
+then mpv's `Using hardware decoding (v4l2m2m-copy)`.
+
+Older versions used a Flatpak override (via nix-flatpak or a Home Manager
+link); `steam-frame-nix-cleanup` removes their entries from it.