Fix iPhone notification audio

[olegberman]

What changed

• replace the @rexa-developer/tiks Web Audio wrapper with one reusable HTMLAudioElement
• unlock playback during the first touch, click, or key interaction and reuse that player for mail and toast sounds
• ship small local WAV files and precache them with the PWA shell
• add audio-unlock and PWA-cache coverage and remove the unused dependency
• include the fix in the existing 0.1.16 release notes

Why

Mobile Safari was not playing the synthesized notification sounds even in the foreground after the user interacted with HQBase. The previous dependency did not reliably satisfy iPhone's media-unlock behavior.

User impact

After the first interaction with HQBase, foreground mail and toast notification sounds can play on iPhone in both Safari and the installed PWA. Reduced-motion remains respected and audio failures remain non-blocking.

Validation

• CI=true WRANGLER_LOG_PATH=/tmp/hqbase-ios-audio-check.log pnpm check
• CI=true WRANGLER_LOG_PATH=/tmp/hqbase-ios-audio-dry-run.log pnpm deploy:dry-run
• CI=true pnpm test:pwa
• workspace node scripts/check-docs.mjs
• git diff --check