docs(audit): cycle 003 README — How it works + Architecture mermaid

[Hesamsamani]

Cycle 003 — Docs/UI refinement

• Add How it works mermaid (Tray → poll providers → gauges → alerts)
• Add Architecture mermaid (Electron main/preload/renderer, providers, SQLite store)
• Update Electron badge to 34 (matches package.json)
• Refresh tagline and nav links

Verification

• 
  pm test: 157 passed
• 
  pm audit: 7 high (transitive: electron-builder chain, form-data, tar) — pre-existing dev-deps; Electron ASAR advisory fixed in >=35.7.5