feat: replace header title/subtitle with live human extinction countdown

[Copilot]

The static header ("AI DEATH CLOCK" + subtitle) is replaced with a live ticking countdown to the human extinction milestone — an immediate visceral hook that contextualises the rest of the dashboard.

What changed

• index.html — Header now contains <h1>HUMAN EXTINCTION COUNTDOWN</h1> and a 5-unit countdown widget (YRS : DAYS : HRS : MINS : SECS) with a caption linking to the milestones section
• src/js/02-counter.js — updateExtinctionCountdown() finds the extinctionMarker: true milestone (77 quadrillion tokens), computes remaining time via getCurrentTokens() + getDynamicRate(), and writes to the five counter elements; module-level constants avoid per-tick allocations
• src/js/21-boot.js — Called once at init (no blank flash) and every second via the existing badge/alert interval
• styles/hero-tabs.css — Orbitron monospace values in accent colour with glow, small uppercase unit labels, responsive down to 480 px; .site-subtitle removed
• tests/e2e/death-clock.spec.js — Header test updated to assert the new title and that #extYears resolves to a positive integer

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 9b8d2090-041c-4f54-b655-046fcbb7e268

📥 Commits

Reviewing files that changed from the base of the PR and between 6e676fb and 37188ea.

📒 Files selected for processing (5)

• index.html
• src/js/02-counter.js
• src/js/21-boot.js
• styles/hero-tabs.css
• tests/e2e/death-clock.spec.js

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch copilot/simplify-death-clock-countdown

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (99df9d1) to head (37188ea).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #92   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines          242       242           
  Branches       109       109           
=========================================
  Hits           242       242           

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