README: full user documentation again, docs/ only technical

The README is the complete user documentation again: features, install,
two sessions, usage, the full options table (and renamed options), every
feature with problem, usage, limitations, security and "remove when",
UI patches at user level (DevTools on the LAN, after a Steam update),
changes outside Nix, rollback and uninstall.

docs/ keeps only the technical side (how the patches work, writing
patches, signatures and the update procedure, the SteamVR debugger
mechanics, cleanup and installer internals), linked from each feature.
docs/options.md, two-sessions.md and desktop-integration.md are gone
(their content is in the README).

docs/launcher-menu.md

@@ -1,78 +1,44 @@
-# Launcher menu
+# Launcher menu: how it works
 
-The VR dashboard's "+" menu (non-Steam programs). Options:
-[options.md#launcher-menu](options.md#launcher-menu).
+Technical details of the "+" menu features. What they do and how to
+configure them: README, [launcher menu](../README.md#launcher-menu-launchermenu),
+[hidden apps](../README.md#hidden-apps-launchermenuhiddenapps),
+[icon fallbacks](../README.md#icon-fallbacks-launchermenuiconfallbacks).
 
 ## Menu patches
 
-`launcherMenu.*`.
+`launcherMenu.*` (module `launcher-menu`): [UI patches](ui-patches.md) in
+Steam's `SharedJSContext` (`modules/launcher-menu/`), each registered only
+when its option is set and reverted by its unpatch when unset (next switch):
 
-**Problem:** the menu is in random order with "Desktop" somewhere in a
-scrolling list, and a click shows no feedback until the window appears, so
-programs often get started twice.
+- `order/`: sorts `ScanForInstalledNonSteamApps()` by name (Steam lists the
+  programs in GLib hash-table order);
+- `pinned-desktop/`: hides Desktop in the list and pins a proxy above/below
+  it;
+- `launch/`: wraps `LaunchNonSteamApp` (menu-only) to close the menu and/or
+  debounce repeated launches;
+- `grid/`: restyles Steam's own items as tiles, which is why launching,
+  sounds and controller navigation keep working;
+- `show-all/`: empties the list Steam hides without Developer Mode.
 
-**What it does:**
-
-- `sort`: programs sorted by name (case-insensitive), Desktop included.
-- `pinDesktop = "top"` / `"bottom"`: Desktop pinned above/below the list,
-  always visible.
-- `closeOnLaunch`: the menu closes on click.
-- `launchDebounceSeconds = <seconds>`: a repeat launch of the same command
-  within that time is ignored (and logged); a program that exits right away
-  can only be restarted once the time is up.
-- `grid.enable`: the programs section becomes a grid of tiles (icon, name
-  below); "Add desktop window" stays a list. Only restyles Steam's items, so
-  launching, sounds and controller navigation keep working. The popup is
-  300 px wide, so `grid.columns` sets the tile size (3 ≈ 92 px, 4 ≈ 68 px,
-  5 ≈ 53 px); `grid.maxRows` limits visible rows, the rest scrolls.
-- `showAllApps`: without Developer Mode Steam hides `konsole`,
-  `systemsettings`, `dolphin`, `plasma-discover`, `vlc`, `firewall-config`,
-  `cmake-gui`, `qrenderdoc`, `lxterminal` and `sh`; this lifts that filter
-  only, so Developer Mode (sshd, xrdp, LAN DevTools forwards) can stay off.
-  Hide single programs with [`hiddenApps`](#hidden-apps).
-
-**Steam Developer Mode** (a Steam setting, not managed here) also makes the
-menu list every desktop entry; `showAllApps` does the same without it.
-
-**Limitation:** the pinned Desktop works with the laser but not with
-thumbstick / D-pad navigation.
-
-**How it works:** [UI patches](ui-patches.md) in Steam's `SharedJSContext`.
-All revert when turned off (next switch). The anchors (APIs, React props,
-CSS) are verified by the offline checker. Tested with Steam client
-1790377368.
+Their anchors (APIs, React props, CSS) are in `modules/lib/signatures.json`
+and verified by the [offline checker](ui-patches.md#after-a-steam-update).
 
 ## Hidden apps
 
-`launcherMenu.hiddenApps`: desktop entry ids (no `.desktop`).
-
-**Problem:** with Developer Mode or `showAllApps`, the "+" menu lists every
-desktop entry, including system tools.
-
-**Fix:** a user entry with `Hidden=true` in `~/.local/share/applications`
-masks the system one (also in the KDE menu). The "+" menu always hides
-`steam` and `vrurlhandler`; for Konsole in VR use
-[`showAllApps`](#menu-patches).
+`launcherMenu.hiddenApps` (module `hidden-apps`): a Home Manager-linked
+desktop entry with `Hidden=true` per id in `~/.local/share/applications`,
+which masks the system entry of the same id for Steam and KDE alike.
 
 ## Icon fallbacks
 
-`launcherMenu.iconFallbacks.enable` (on by default), `iconFallbacks.extra`.
-
-**Problem:** Steam resolves `Icon=` only in the hicolor theme (and
-`pixmaps`), so Konsole and KDE System Settings, whose icons only Breeze has,
-show without icon.
-
-**Fix:** Home Manager links nixpkgs' Breeze SVGs of `utilities-terminal`
-and `preferences-system` (plus `extra`) into
-`~/.local/share/icons/hicolor/scalable/apps/`; a name Breeze doesn't have
-fails the build, `enable = false` provides none. When the set of links
+`launcherMenu.iconFallbacks.*`: Home Manager links nixpkgs' Breeze SVGs
+into `~/.local/share/icons/hicolor/scalable/apps/`. When the set of links
 changes, the switch bumps the mtime of `~/.local/share/icons/hicolor`, so a
 running Steam rescans (GTK only rereads a theme whose directory changed).
-Each switch also prints hints: icons of programs Steam can't find that
-Breeze has (add them to `extra`), and fallbacks hicolor has anyway.
+The switch's hints come from `icon-fallbacks.sh`.
 
 **Migration:** until 2026-09 a script made these links on switch and listed
 them in `~/.local/state/steam-frame-nix/icon-fallbacks`; the first switch
 replaces them with Home Manager's and `steam-frame-nix-cleanup` removes the
-rest. `iconFallbacks` used to be a list; a list now fails with a hint (use
-`extra`, or `enable = false` for `[ ]`).
+rest.