feat(ui): add a home button to the status bar, rename launcher to home

Leaving an app was the app's own responsibility, so one that shipped without
an exit could only be escaped with a reset. The bar now paints a back button
into its leading square and the firmware treats that rect as home, acting on
release so a press sliding into the app cancels. The app it returns to is
/apps/home, which is what it is to the user.

Card grids in home and settings centre left to right as a unit.

docs/lua-api-parity.md

@@ -34,7 +34,7 @@ strength a status screen wants. Its `state` vocabulary is a subset: crosspoint r
 | Themes | none | `sys.getTheme/setTheme`, `/lib/theme.lua` | Colour panel. |
 | Rotation | `gui.setOrientation("portrait")` | `sys.setRotation(degrees)` persisted, `gui.setRotation(0-3)` for one frame | This device stores rotation in settings and remaps touch to match. |
 | Clock | nothing exposed; UTC offset is a C++ setting | `sys.clockSynced`, `sys.getTimezone/setTimezone` with POSIX TZ rules | Timezone here is a stored rule, so `os.date()` returns local time with DST handled by libc. |
-| Launching | launcher is C++ | `sys.launch(path)`, launcher is a Lua app | The launcher is just another app here. |
+| Launching | launcher is C++ | `sys.launch(path)`, home is a Lua app | The launcher is just another app here, named `home`. |
 | Modules | single-file apps; `require` unusable | `require` works, `package.searchers` reads the SD card, `/lib` on the path | Shared code such as `ui.lua` needs it. **crosspoint should adopt this.** |
 | BLE | `ble.*` | none | No BLE use case here yet. |
 | TLS memory | `TlsScratchLoan` lends the framebuffer to wolfSSL | none | crosspoint is heap-starved; this device has ~280KB free. |