feat(ui): black and red redesign with module thumbnails

[pawel120]

New look for the app: black background with red accents, inspired a bit by the PWr Racing Team style. Layout and features stay the same as on main, this is mostly a visual change. Font is Barlow, bundled locally through @fontsource so it works with the CSP.

What changed:

• Shell: title bar with the logo and "Modular Brain-Computer Interface", navigation with icons on the left (red bar on the active page), app version at the bottom of the sidebar, device panel on the right.
• Dashboard: module cards with a thumbnail, name and version, description, category / "EEG required" tags and a red Open button.
• Modules and devices can have thumbnails now. Modules already had appearance.thumbnail in the manifest, I added thumbnails for Cortex, Miner and Neuorrun in public/modules/<id>/thumbnail.webp. Devices got an optional thumbnail field in DeviceInfo. If there's no image (or it fails to load) the card just shows without it.
• Device page: devices are shown as cards in the same style as the modules.
• Cortex, Miner and Neuorrun recoloured to match. Brain shaders are kept, only the colour ramp was tuned. The SSVEP stimulus in Miner (squares, sizes, colours, frequencies) is untouched.
• Welcome screen: only colours changed, content and timing are the same.
• In dev builds there are simulated devices (the existing SimulationEEGAdapter and a new SimulationBrainMetricsAdapter), so every module can be opened without a headset. They're not registered in production.

Cortex fixes:

• Rotation was stuttering a lot. The Welch PSD step took 5-11 ms every ~100 ms on the main thread, so the signal processing now runs in a Web Worker.
• The wireframe overlay draws ~750k lines per frame on the medium mesh, which is very slow in WebView2, so it's not drawn anymore.
• The camera used to roll the head sideways when dragging horizontally. Now it turns around the vertical axis, and resizing the window doesn't reset the view.

Things to look at:

• The module thumbnails are placeholders cut from a mockup. If someone has real screenshots or artwork, just replace the files, nothing else needs to change.
• main is already merged into this branch, so there shouldn't be conflicts.

Tested:

• typecheck, lint, prettier, tests (95/95) and build pass locally.
• Clicked through everything in the browser with the simulated devices.
• Not tested yet with real BrainAccess / BrainLink or in the Tauri window, so it would be good to run the end-to-end flow from the README before merging.