feat: Atom UI polish and feature parity

[mattgodbolt]

Now that the Atom boots (#660), there are several UI/polish items remaining before removing the VITE_ATOM_ENABLED gate and making Atom publicly available.

UI elements from original PR #505 not yet integrated

• NEC-TV monitor bezel (replaces CUB monitor for Atom, image at public/images/80s-nec-tv.png)
• Cassette transport controls (play/stop/rewind buttons)
• SDCard menu (load/download/clear MMC ZIP images)
• PPIA register display in debugger panel
• Hide BBC-only menu items when Atom is selected (65c02 coprocessor, Teletext, Music5000, Econet)
• Hide Atom-only items when BBC is selected

Configuration

• Noise killer toggle (reduces video snow, runtime config not model property)
• Atom-specific keyboard layout selection

Gate removal

• Remove VITE_ATOM_ENABLED gate in models.js once everything is ready
• Ensure config.setModel handles unknown models gracefully (for atom* hostname when gate is active)

Assets

• Decide whether MMC ZIP archives (16MB+) should be in the repo, fetched at runtime, or hosted separately
• Add public/discs/atom/disk0725.dsk default disc image

Credits

• Update README with Atom support description and CommanderCoder attribution
• Document Atom URL parameters in README

[mattgodbolt]

@CommanderCoder a list I meant to link earlier...