✨[Feat] Add favicon and social preview polish - #66

Summary

• add same-origin favicon, Apple touch, and 1200 × 630 social preview assets
• add favicon declarations to every static page and OG/Twitter metadata to every canonical page
• keep localized title and description metadata synchronized during language changes
• extend resource, metadata, image-dimension, and root-relative path regression coverage

Validation

• git diff --check 2bd877d9a5fda6f5041ea83e375a1003dbcc11fa..HEAD
• node tests/run-all.mjs
• local HTTP audit: homepage, direct PDF Merge route, four icon files, and OG image returned HTTP 200

Manual QA status

Automated browser control is blocked in the current Windows environment before a browser session can initialize (failed to write kernel assets: path not found). The PR remains draft pending a real-browser check of tab favicon, direct tool navigation, light/dark themes, console, and Network requests.

Release policy

• no version bump
• no tag or GitHub Release created
• existing v2.0.0 pre-release state unchanged