feat: per-session ringtone, mute toggle, countdown, and premium UI redesign

[princechrix]

Summary

• Per-session ringtone (MP3/WAV/OGG) with looping playback during nudge overlay
• Mute/unmute toggle on session cards with persisted state
• Live countdown with progress bar on active session card
• Complete UI redesign: warm gold accent, SVG icons, atmospheric overlay with ring countdown
• macOS DMG build support alongside Windows NSIS
• Updated README with installation instructions for both platforms

Changes

• Ringtone system: custom nudger-media:// protocol, native file picker, looping audio on overlay, stops on dismiss
• Mute toggle: SVG speaker icon per card, respects muted flag when nudge fires, persisted across restarts
• Countdown: nudge engine tracks nextNudgeAt timestamp, renderer shows progress bar + MM:SS timer on active card
• UI redesign: warm gold color system, full SVG icon set, grouped form fields (Identity/Timing/Sound), card accent bar, overlay with SVG ring countdown + grain texture + staggered entrance animation
• Build config: added macOS DMG target, platform-specific build scripts (build:win, build:mac)
• README: installation sections for Windows and macOS, updated feature list and project structure
• Design brief: added DESIGN_BRIEF.md for UI/UX reference

Testing

• Create a session with ringtone — verify file picker and playback
• Toggle mute — verify icon changes and next nudge respects it
• Start session — verify progress bar fills and countdown ticks
• Nudge fires — verify overlay ring countdown animates, then ack area appears
• Dismiss — verify audio stops and overlay closes
• Windows build: pnpm build:win produces installer
• macOS build: pnpm build:mac produces DMG (requires macOS)