feat(release): v2.1.2 — CI green: all 5 workflow fixes + .deb

## CI Fixes
- fix(lint): check:docs-sync — bump docs/openapi.yaml version to 2.1.2 + add [Unreleased] to CHANGELOG
- fix(ci): npm-publish.yml — use 'npm ci --ignore-scripts' to skip prepublishOnly during install, then run prepublish.mjs explicitly with JWT_SECRET; prevents double-build loop that caused all npm CI publishes to fail
- fix(ci): docker-publish.yml — replace two-job digest approach (required ubuntu-24.04-arm, unavailable on public repos) with single-job QEMU-based multi-arch build (linux/amd64 + linux/arm64 on standard ubuntu-latest)
- fix(ci): electron-release.yml — add .deb target to Linux electron-builder, collect .deb files in release-assets step, attach *.deb to GitHub release assets

## Documentation
- fix(docs): README.md language bar — fix 29 broken links (README.<lang>.md → docs/i18n/<lang>/README.md)
- fix(docs): docs/i18n/*/README.md — update back-links to ../../README.md and cross-links between languages
- fix(docs): electron/package.json — add deb target (x64+arm64) to linux build config

docs/openapi.yaml

@@ -1,7 +1,7 @@
 openapi: 3.1.0
 info:
   title: OmniRoute API
-  version: 2.0.0
+  version: 2.1.2
   description: |
     OmniRoute is a local-first AI API proxy router. It provides an OpenAI-compatible
     endpoint that routes requests to multiple AI providers with load balancing,