[playlists] Review the temporary playlist on phones

[frankrousseau]

Problem

• The temporary playlist modal (My Checks "Build playlist") is unusable on phones: the bar overflows, the video gets no height in landscape, the comment panel is hidden and there is no close button
• My Checks breaks on landscape phones, wider than the 768px breakpoint: desktop table, list squeezed to a few pixels under the header

Solution

• Phone mode for the temporary player (portrait and landscape): compact entity strip, video, one-line bar (prev/next, play, frame, undo, pencil, full screen), comment panel shown by default
• In-page full screen, low definition, and no export, revision publishing or assignees on phones
• Modal covers the whole screen above the topbar, with a close button, and hides the support chat while open
• Landscape phones get the phone layout of My Checks (read-only cards, page scroll), a 44px topbar and a compact header