Add "Download missing" action to the platform list

[nixxou]

What

Adds a Download missing action to the home platform list. With a platform focused, R1 downloads every game of that platform that isn't on the device yet.

• Gathers the not-yet-downloaded games for the focused platform (from cache; refreshes from the server if the cache is empty)
• Confirmation modal: Download N missing game(s) for X?
• Hands off to the existing DownloadScreen → DownloadManager, so the progress modal, cancel, and artwork/unzip/gamelist handling are all reused
• Result modal: Downloaded N of M game(s).

Why

Today you can only download one game at a time, or hand-pick a multi-selection in the game list. "Give me everything new for this system" means a lot of scrolling and tapping; this is one gesture from the screen you're already on.

Notes / decisions

• Key: R1 via TertiaryActionButton — free on this screen and present on Miyoo (Select is taken by reorder).
• Kid mode: action + footer hint hidden.
• No per-row cost: the missing set is computed only on the R1 press, for the one focused platform. The home list's load path is unchanged — nothing scans every platform's library on open.
• "Collections" row: no-op (just redraws the list).
• i18n: active.en.toml updated. Parameterised strings use %d/%s + fmt.Sprintf (same as ui/bios_download.go) so they render correctly even for locales that haven't picked up the new keys yet. Happy to run task i18n / add translations_todo entries if you'd prefer that in-PR.
• Follows existing patterns: executeMultiDownloadUI / executeDownloadUI in app/helpers.go, action enum + transition case in app/transitions.go.

Testing

• go build ./..., cross-compiled for muOS (arm64) via Docker — green
• go test ./ui/... — new TestMissingGames (single-file / multi-file / missing-slug)
• On-device (muOS, French locale): confirm modal, progress + cancel, files land in the mapped rom dir, cursor restored after, kid mode hides it

Per the contributing guide I'm happy to take the UX discussion to Discord first (key choice, whether this also belongs on the collection→platform screen) — just let me know.