Backport #40104 by @0xJacky
Remove the `body {margin: 0; padding: 0}` reset from
https://github.com/go-gitea/gitea/pull/38396, which left HTML mails
flush against the edge in Apple Mail and Thunderbird. The devtest
preview stops injecting its own padding, so it matches delivered mails.
These clients render the mail as a document and apply the default 8px
body margin again, as with GitHub and GitLab notifications. Webmail
clients replace `<body>` and keep their own inset.
Fixes: https://github.com/go-gitea/gitea/issues/40103
Assisted-by: Codex:GPT-6.1, Claude:Opus 5.5
Co-authored-by: Jacky <jacky-943572677@qq.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: silverwind <me@silverwind.io>
### Description
Replaces all remaining direct `gopkg.in/yaml.v3` imports with
`go.yaml.in/yaml/v4` across models, modules, routers, services, and
integration tests. `gopkg.in/yaml.v3` moves from a direct to an indirect
dependency in `go.mod`.
#### API compatibility
The yaml.Node type, node.Kind/node.Content traversal style
(modules/markup/markdown/convertyaml.go), and the
UnmarshalYAML(*yaml.Node) interface signature
(modules/optional/serialization.go) are all preserved in v4 — no
call-site changes were required beyond the import path.
**Related:**
- https://github.com/go-gitea/gitea/pull/36564#issuecomment-4526536805
---------
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.8) <noreply@anthropic.com>
