Polish mobile proof and add responsive docs diagrams

[TeoSlayer]

Summary

• give the live mobile captures a proper inner bezel and full-width phone layout
• make every install command readable and untruncated on small screens
• add responsive Mermaid diagrams across the overview, app, mobile, Refstream, reliability, security, and self-hosting guides
• load Mermaid only on documentation pages and use separate desktop/mobile graph directions
• validate diagram content and keep the production dependency audit clean

Validation

• npm run check (96 tests)
• npm run build:web
• npm audit --omit=dev
• Playwright at 390x844 and 1440x1000; no horizontal page overflow and all diagrams rendered