chore: cut GitHub, make netswitch Gitea-only

- relink README/CHANGELOG/pyproject/SECURITY to git.engelgardt23.ru
- update-check + badges point to Gitea; rename GITHUB_REPO -> REPO
- port CI to .gitea/workflows (self-contained, publishes Gitea release)
- remove .github (workflow + issue templates); drop made-by lines

CONTRIBUTING.md

@@ -6,9 +6,8 @@
 
 ```
 netswitch/
-├── .github/
-│   ├── workflows/release.yml          ← CI: tag-driven build + GitHub Release
-│   └── ISSUE_TEMPLATE/                ← bug / feature / security routing
+├── .gitea/
+│   └── workflows/release.yml          ← CI: tag-driven build + Gitea Release
 ├── src/
 │   └── netswitch.ps1                  ← the whole tool (~150 lines)
 ├── CHANGELOG.md                       ← Keep a Changelog format, newest first
@@ -43,16 +42,16 @@ Invoke-ps2exe -inputFile src/netswitch.ps1 -outputFile netswitch.exe `
 4. Tag: `git tag vX.Y.Z`.
 5. Push: `git push && git push --tags`.
 
-GitHub Actions picks up the tag, builds the exe via ps2exe, writes the SHA-256, and creates the GitHub Release with the zip attached.
+Gitea Actions picks up the tag, builds the exe, writes the SHA-256, and creates the Gitea Release with the zip attached.
 
 ## Where features go
 
 The whole tool is a single PowerShell file with these sections (in order):
 
-1. `$NetswitchVersion` / `$GithubRepo` — top of file.
+1. `$NetswitchVersion` / `$Repo` — top of file.
 2. **Self-elevate** block — UAC if non-admin.
 3. **Banner** — Write-Host title.
-4. **`Test-NetswitchUpdate`** — GitHub /releases/latest poll.
+4. **`Test-NetswitchUpdate`** — Gitea /releases/latest poll.
 5. **Adapter filter + picker** — `$skipDescriptionPattern`, `$skipMediaTypes`, `Get-NetAdapter | Where-Object {...}`.
 6. **Mode prompt** — Static / DHCP.
 7. **Static branch** — `netsh interface ipv4 set address ... static ...`.