Closes #695.

## Summary
- Bump Hoppscotch from 2026.5.0 to 2026.6.0.
- Update the Docker Hub Hoppscotch AIO image tag, README upgrade notes,
production docs, and image assertion tests.
- Remove the decorative marker from the README security scan heading
while touching the file.

## Upstream Evidence
- GitHub release:
https://github.com/hoppscotch/hoppscotch/releases/tag/2026.6.0
- Official release blog:
https://hoppscotch.com/blog/hoppscotch-v2026-6-0
- Docker Hub image manifest verified:
`docker.io/hoppscotch/hoppscotch:2026.6.0`
- Manifest platforms: `linux/amd64`, `linux/arm64`.
- The upstream cloud rollback note applies to Hoppscotch Cloud only;
release notes state self-hosted deployments are not affected.
- 2026.6.0 adds OAuth2 `id_token` support, mock server URL handling for
subpath deployments, Thai language support, self-hosted admin validation
improvements, and security hardening.

## Site Sync
- Site PR: helmforgedev/site#376

## Validation
- `make image-verify IMAGE=docker.io/hoppscotch/hoppscotch:2026.6.0`
- `make deps-check CHART=hoppscotch`
- `helm unittest charts/hoppscotch` passed: 14 suites, 81 tests.
- `node scripts/charts/template-standards-check.js --chart hoppscotch`
- `make standards-check CHART=hoppscotch`
- `make validate-chart CHART=hoppscotch` passed fully: 18 layers,
including k3d behavioral validation for default and all CI values
scenarios.
- `make site-sync-check CHART=hoppscotch`
- `make release-check REPO=charts`
- `make attribution-check REPO=charts`

## Lab Note
- An earlier k3d run failed because the Docker Desktop VM/host disk was
out of space and the API server timed out. After clearing generated/temp
files and restarting Docker Desktop, `make validate-chart
CHART=hoppscotch` passed end-to-end.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
  * Updated the deployed Hoppscotch version to `2026.6.0`.
* **Documentation**
* Refreshed Helm chart notes, including default image tag details and
upgrade notes for the latest release.
* Updated production guidance for the default proxy URL hardening to
reference `PROXY_APP_URL`.
* **Tests**
* Updated deployment assertions to verify the `2026.6.0` Hoppscotch
image tags across the main container and init containers.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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