feat: add Site Vital Signs patient monitor page

[arc-butler]

Adds a /vitals/ page that presents website metrics as a CRT patient monitor — complete with animated EKG waveform, phosphor-green display, and medical diagnosis.

What

• _pages/vitals.md — New page at /vitals/ with Liquid-injected site data
• assets/js/vitals.js — Vanilla JS engine: parses site metrics, computes vitals (heart rate from post frequency, temperature from recency, SpO2 from content freshness, etc.), renders a real-time EKG on canvas
• _sass/_vitals.scss — CRT monitor styling: scan lines, phosphor glow, dark theme

Why (weird-but-sensible)

• Sensible: At a glance you see site health — posting cadence, content freshness, activity level
• Weird: Presented as an actual 80s CRT patient monitor with green phosphor display, animated EKG with PQRST complexes, medical diagnosis, and cycling alerts
• Zero dependencies: Vanilla JS, all styling self-contained
• Uses existing site data (posts, projects, travel, books) via Liquid injection

Built

• Claude Code was unavailable (expired auth), built directly with same spec
• JS syntax validated, frontmatter validated, SCSS import wired

[ci skip]