fix: bugs in new landing page components (LiveDemo, ROICalculator, SEO pages) #4745

Originally written by @ccantynz-alt on GitHub.
Imported from https://github.com/ccantynz-alt/voxlen/pull/56

Summary

• LiveDemo transcript bug: prev + final || interim evaluated as (prev + final) || interim due to operator precedence — when final was empty the transcript was silently reset to just the interim result instead of accumulating. Fixed to prev + (final || interim).
• ROICalculator NaN/Infinity: sessionsPerDay = (hoursPerDay * 60) / typingMinutes could divide by zero, producing Infinity → NaN down the chain → rendered "NaN×" in the UI. Added typingMinutes > 0 guard and isFinite check.
• SEOPage <title> JSX noop: <title> rendered as JSX has no effect on the actual browser tab or search result title. Replaced with useEffect(() => { document.title = ... }).
• Intel Mac download: detectPlatform() returned "mac-arm" for all Macs, so Intel Mac users got the wrong download binary. Now detects Intel via UA string /Intel/i or /x86_64/i.

Test plan

• TypeScript clean (npx tsc --noEmit)
• LiveDemo: speech recognition transcript accumulates correctly across multiple finals/interims
• ROICalculator: no "NaN×" renders at min input values
• SEO pages: browser tab title updates when navigating to /voice-dictation-software etc.
• Intel Mac: detectPlatform() returns "mac-intel" on Intel hardware

https://claude.ai/code/session_011MFNHeFKrhfuSrEYVgrdyr

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