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/launcher-menu.md

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+# Launcher menu
+
+The VR dashboard's "+" menu (non-Steam programs). Options:
+[options.md#launcher-menu](options.md#launcher-menu).
+
+## Menu patches
+
+`launcherMenu.*`.
+
+**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.
+
+**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.
+
+## 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).
+
+## 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
+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.
+
+**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 `[ ]`).