Prevent response modifiers that require body rewriting from running when
the body rewrite gate blocks buffering (for example, chunked transfer
encoding).
Add an explicit `requiresBodyRewrite` capability and implement it for
HTML/theme/error-page modifiers, including bypass delegation.
Also add a regression test to ensure the original response body remains
readable and is not closed prematurely when rewrite is blocked.
This commit fixeds the "http: read on closed response body" with empty page error
happens when body-rewriting middleware (like themed) runs on responses where body rewrite is blocked (e.g. chunked),
then the gate restores an already-closed original body.
This is a large-scale refactoring across the codebase that replaces the custom
`gperr.Error` type with Go's standard `error` interface. The changes include:
- Replacing `gperr.Error` return types with `error` in function signatures
- Using `errors.New()` and `fmt.Errorf()` instead of `gperr.New()` and `gperr.Errorf()`
- Using `%w` format verb for error wrapping instead of `.With()` method
- Replacing `gperr.Subject()` calls with `gperr.PrependSubject()`
- Converting error logging from `gperr.Log*()` functions to zerolog's `.Err().Msg()` pattern
- Update NewLogger to handle multiline error message
- Updating `goutils` submodule to latest commit
This refactoring aligns with Go idioms and removes the dependency on
custom error handling abstractions in favor of standard library patterns.
- Introduced a new themed middleware that allows for dynamic theme application.
- Added support for multiple themes: dark, dark-grey, solarized-dark, and custom CSS.
- Included CSS files for each theme and a font CSS template for font customization.
- Updated middleware registry to include the new themed middleware.
