Refine UI layout, restyle sliders/scale keys, and add undo/redo for orbs & gate speed

[jeromepotter]

Motivation

• Make UNDO/REDO capture all meaningful state changes (including deployed ORBs and gate speed) and provide REDO support.
• Move PLAY/ORB controls into the top-right HUD while keeping gate controls under the sequencer and move UNDO/REDO/CLEAR into the Settings modal to declutter the main UI.
• Restyle sliders to a pill-track/thumb design and simplify the custom scale selector so selected notes are black and unselected are white to remove confusing gray sharps/flats.

Description

• UI/layout: moved PLAY and ORB buttons into the top-right HUD, reorganized HUD into .hud-group blocks, and reflowed bottom controls into centered .controls-row and a separate gate row under the sequencer.
• Settings modal: relocated UNDO/REDO/CLEAR into the #global-panel as full-width buttons and added btn-undo / btn-redo handlers.
• Undo/Redo: added a redoStack, implemented redoLastMove() and updateUndoRedoButtons(), expanded createUndoSnapshot() to include projectiles and gate spin state, cleared redoStack on new pushes and pattern loads, and pushed an undo snapshot when spawning an orb and when toggling/adjusting gate spin.
• Slider & scale UI: introduced CSS variables and new slider styling (pill track + slim rounded thumb) to match the requested look, removed the prior black-key special-case and updated .key-circle styles so active notes are black and unselected are white.
• Gate speed undo grouping: start a single undo push when user begins adjusting gate speed and clear that grouping on change/pointerup/touchend/blur to avoid noisy undo history.

Testing

• Launched a local static server and executed a Playwright script that loaded index.html and produced a full-page screenshot, which completed successfully and produced artifacts/les-cercles-ui.png.
• No other automated tests were added or executed beyond the screenshot verification.

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Codex Task

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