docs: README as overview + options, one docs page per feature

README keeps intro, feature list linking docs/, install, two sessions,
usage, the complete options table, changes outside Nix, rollback,
uninstall. Each feature's details (problem, what you get, configuration,
limitations, how it works) move to its own page in docs/; docs/dashboard.md
is split per feature and docs/changes-outside-nix.md becomes
docs/cleanup.md.

docs/firefox.md

@@ -1,36 +1,81 @@
-# Firefox: how it works
+# Firefox
 
-`firefox.*` (module `firefox`). What it does and how to configure it:
-README, [Firefox](../README.md#firefox-firefox).
+`firefox.*`, module `firefox`. Options:
+[README, Options](../README.md#options).
 
-## Default prefs
+## Problem
 
-`prefs` and the fixes are *default* values (`pref()`), not user values:
-`about:config` can still change them per profile, and nothing is written to
-`prefs.js`, so removing one leaves nothing behind. Firefox reads default
-prefs from `defaults/pref/*.js` in its system config dir, which in the
-Flatpak is `/app/etc/firefox`, the mount point of the
+In the Steam session gamescope never shows fullscreen windows, so Firefox
+looks frozen when a page goes fullscreen; sites send AV1, which the Frame
+decodes in software; and the two sessions can't see each other's Firefox,
+so a second instance stops at the locked profile.
+
+## What you get
+
+A launcher for the Flathub Firefox Flatpak (`org.mozilla.firefox`, stable;
+install it yourself). The launcher shadows the Flatpak's own entry (same
+ID), so default-browser associations keep working.
+
+- **`vrFullscreenFix`** (on): `full-screen-api.ignore-widgets` makes
+  fullscreen fill just the window. Not applied in the desktop profile.
+- **`disableAv1`** (off): `media.av1.enabled = false`. The Frame's decoder
+  driver (`iris`) has no AV1, only H.264, HEVC and VP9, so YouTube and co.
+  send VP9/H.264, decoded in hardware, instead of software AV1.
+- **`prefs`:** further `about:config` values for every profile; they can
+  also override the fixes above.
+- **`desktopProfile`** (`"desktop"`): in the nested desktop the launcher
+  uses this separate profile (a normal Firefox profile with its own browser
+  data, created on first use). `null`: the default profile in both sessions.
+
+`prefs` and the fixes are *default* values, not user values: `about:config`
+can still change them per profile, and removing one leaves nothing behind.
+Changes take effect at the next start of Firefox.
+
+## Configuration
+
+```nix
+steamFrame.firefox = {
+  enable = true;
+  disableAv1 = true;
+  prefs."browser.startup.page" = 3;   # restore the previous session
+};
+```
+
+## Caveats
+
+- A `user.js` of your own in the desktop profile is never touched (the
+  fullscreen fix then stays on there).
+- A `org.mozilla.firefox.systemconfig` Flatpak extension of your own would
+  conflict with the one this module provides.
+- **Remove** `vrFullscreenFix` **when** gamescope shows fullscreen X11
+  windows in VR, `disableAv1` **when** a SteamOS kernel adds AV1 to `iris`.
+
+## How it works
+
+### Default prefs
+
+The prefs are written with `pref()`, and nothing goes to `prefs.js`.
+Firefox reads default prefs from `defaults/pref/*.js` in its system config
+dir, which in the Flatpak is `/app/etc/firefox`, the mount point of the
 `org.mozilla.firefox.systemconfig` extension. home-manager provides that
 extension as a link from
 `~/.local/share/flatpak/extension/org.mozilla.firefox.systemconfig/aarch64/stable`
 to a store directory; Flatpak mounts it itself, so the sandbox doesn't get
-`/nix`. (A systemconfig extension of your own would conflict with it.)
+`/nix`.
 
-## Desktop profile
+### Desktop profile
 
-The launcher (a desktop entry with the Flatpak's ID, shadowing its entry)
-picks `desktopProfile` when started in the nested desktop. The desktop
-profile undoes the fullscreen fix only while its Firefox runs: the launcher
-links the profile's `user.js` to
-`/app/etc/firefox/steam-frame-nix-desktop-user.js` (a sandbox path) right
-before starting Firefox, waits for it, and once it has exited and the
-profile is no longer in use removes the link and the value Firefox stored
-from it in `prefs.js`. A second launch that just hands a URL to the running
-Firefox leaves both in place. A `user.js` of your own is never touched (the
-fix then stays on in that profile). After a crash, the next launch or
-`steam-frame-nix-cleanup` (on switch) removes them.
+The launcher (a desktop entry with the Flatpak's ID) picks `desktopProfile`
+when started in the nested desktop. The desktop profile undoes the
+fullscreen fix only while its Firefox runs: the launcher links the
+profile's `user.js` to `/app/etc/firefox/steam-frame-nix-desktop-user.js`
+(a sandbox path) right before starting Firefox, waits for it, and once it
+has exited and the profile is no longer in use removes the link and the
+value Firefox stored from it in `prefs.js`. A second launch that just hands
+a URL to the running Firefox leaves both in place. After a crash, the next
+launch or `steam-frame-nix-cleanup` (on switch) removes them.
 
-## Older versions
+### Older versions
 
 Older versions linked or copied a `user.js` into every profile; those and
 the values they left in `prefs.js` are removed by `steam-frame-nix-cleanup`