UI polish: three.js visualizers, tabbed observation panel, instant startup

[jamiesun]

Refines the workbench UI per the "sonic control room" reference:

• Custom seek bar — pointer-captured slider with hover time tooltip, ticks, keyboard control (arrows/Home/End); replaces the native range input.
• Rebuilt player bar — round transport pod with dashed-ring play button and loop pill, spectrum stage with expand button, visual side panel, segmented stereo LED meters with peak hold.
• Tabbed right panel — ScenePanel split into Observe / Render / Outputs; Observe leads with a 2×2 grid of large 270° instrument gauges (value + semantic word), tabs carry status lights (validation dot, building pulse, output count).
• Four three.js visual styles — flux (particle nebula), terrain (synthwave grid), rings (tunnel), crystal (reactive icosahedron); lazy-loaded chunk (~508 KB) kept out of the startup bundle; WebGL failure falls back to bars without stopping playback.
• Auto style selection — auto.ts derives energy/dynamics/brightness/density traits from the decoded AudioBuffer and picks a style deterministically on each load (9 unit tests).
• Instant open — Tauri window backgroundColor + inline dark shell and EQ boot splash in app.html, faded out on mount; no white flash.
• Fullscreen visualizer — ⛶ expands to a VisualizerOverlay with idle-hiding HUD, style pills, ESC/space/double-click controls.
• Readability + texture pass — app-wide font scale bump (9/10/11px floor), self-drawn select chevrons, glowing range thumbs, button depth states.

Verification: npm run check 0 errors/0 warnings; vitest 11/11; production build green. No Rust source changes (config-only tauri.conf.json).