feat(mobile): keyboard shortcuts, drag and drop, and multi-window on tablets

[altaywtf]

Change

On tablets, a keyboard drives Files and the player, files drag out to other apps, and links and torrents drop in.

Keyboard Shortcuts Helper	A dropped .torrent opens Add transfer

• Delete keeps the menu's confirm rules; Escape never leaves the app.
• A dragged file streams from put.io, readable by the other app only for that drop; the URI carries no token or account id.
• Split-screen panes keep playing, and attaching a keyboard no longer restarts the screen.

Closes #44

Validation

• ./gradlew verify and the four debug assembles
• MobileTabletPowerProofTest on a tablet-sized emulator beside a probe app (captures)
• Unverified: the live API, real drop targets, mouse drags, physical tablets and ChromeOS

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Written by an agent (Claude Code, Opus 5.5)

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[altaywtf]

Tablet proof on bbb7335, API 37 emulator at 1280x800 dp with a hardware keyboard; synthetic (faked Files, local video, in-process download, no API calls). Keyboard run: Tab into Files, Enter into a folder, Backspace up, Ctrl+F, Escape, F5, Delete to Trash, the Keyboard Shortcuts Helper, Enter on a video, Space, Right twice, Escape back to its row.

20261004-223844-tablet-keyboard-shortcuts-final.mp4

[altaywtf]

Split screen and drag and drop: the playing video keeps the same Activity and player when the probe app opens beside it; poster.jpg dragged into the probe (its own process) reads 18,939 bytes only through the drop's grant, with no write and nothing after release; the probe's .torrent and magnet link dropped into put.io open Add transfer, the second asking before replacing the draft.

20261004-223841-tablet-drag-and-drop-split-screen-final.mp4

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[altaywtf]

Drag-out now streams on ff31167: the probe received poster.jpg's 18,939 bytes through ranged reads of the in-process source, with no copy on the device, readable only under the drop's grant, never writable, and not after release. Both MobileTabletPowerProofTest selectors pass again on a freshly booted API 37 emulator.