Start on boot: an opt-in setting for boxes without a firmware autorun #255

Problem

A box that only shows cameras (a wall display, a demo stand) should open Decoder by itself after power-on or a reboot. Today someone has to start it with the remote every time.

Some TV-box firmwares already have this: their app list has a per-app menu (Menu key on a tile) with AUTORUN / CLEAN AUTORUN, and a boot receiver of the firmware starts the chosen app. It works with Decoder as it is (checked on the Aceline box: Decoder comes up ~60 s after boot). Many firmwares and phones have nothing like it, and Decoder itself has no way to start on boot.

Proposal

An opt-in Start on boot setting in Decoder, off by default:

1. A BOOT_COMPLETED receiver (RECEIVE_BOOT_COMPLETED permission). When the setting is on, it opens Decoder with the saved slots and view (grid, U cell, slides) as on a normal start.
2. Android 10+ blocks activity starts from the background. The documented exception is an app allowed to draw over other apps (SYSTEM_ALERT_WINDOW). When the user turns the setting on and the app lacks it, Decoder opens the system "Display over other apps" screen; where the firmware has no such screen (many TV boxes), it shows the one-line adb command instead:
   adb shell appops set com.openipc.decoder SYSTEM_ALERT_WINDOW allow
3. The checkbox lives in Settings. The menu grid keeps its shape (Menu as an 8 / 4 / 4 grid with words: slots on top, Settings WebUI Grid Slides, Exit Reset Debug UDP #177).
4. /stats.json reports the setting and whether the permission is granted, so a box can be checked remotely.
5. README (all three languages): a short "Start on boot" section — use the firmware's own autorun where there is one, otherwise this setting.

Out of scope

• Making Decoder the home screen (a HOME category). It is a stronger lock-in and needs the user to pick the default launcher; a separate issue if needed.
• Waking the box from standby: that is up to the firmware and its boot loader.

Test

• A box without a firmware autorun and a phone: setting on → reboot → Decoder opens by itself with the saved view; setting off → it does not.
• Android 10+ without the overlay permission: the hint appears, nothing crashes on boot.
• A box with a firmware autorun: no double start when both are on.