chore: misc go 1.27 tweaks (#39069)

Follow-up to https://github.com/go-gitea/gitea/pull/39068, which
disabled `modernize` entirely.

- re-enable `modernize`, with only the new `embedlit` rule disabled. It
flattens embedded struct literals across ~145 files, and orphans imports
in 6 of them that the fixer does not remove
- apply the rest of the suite: `errors.AsType`, `reflect.TypeAssert`,
`strings.Cut`, and dropping the legacy import comment
- use the new stdlib `uuid` package, `github.com/google/uuid` becomes
indirect
- use `strings.CutLast` in place of manual `LastIndex` slicing in label
scopes, email domains and the diff tree list
- take the header lint skip dirs from the `go.mod` `ignore` directive
and skip dot-directories, instead of hardcoding the list

Assisted-by: Claude Code:claude-opus-5

go.mod

@@ -60,7 +60,6 @@ require (
 	github.com/google/go-github/v89 v89.0.0
 	github.com/google/licenseclassifier/v2 v2.0.0
 	github.com/google/pprof v0.0.0-20260802141513-ef3492d7dac3
-	github.com/google/uuid v1.6.0
 	github.com/gorilla/feeds v1.2.0
 	github.com/gorilla/sessions v1.4.0
 	github.com/hashicorp/go-version v1.9.0
@@ -194,6 +193,7 @@ require (
 	github.com/google/flatbuffers v25.12.19+incompatible // indirect
 	github.com/google/go-querystring v1.2.0 // indirect
 	github.com/google/go-tpm v0.9.8 // indirect
+	github.com/google/uuid v1.6.0 // indirect
 	github.com/gorilla/css v1.0.1 // indirect
 	github.com/gorilla/mux v1.8.1 // indirect
 	github.com/gorilla/securecookie v1.1.2 // indirect
@@ -276,6 +276,9 @@ require (
 ignore (
 	./.venv
 	./node_modules
+	./public
+	./vendor
+	./web_src
 )
 
 // When doing "go get -u ./...", Golang will try to update all dependencies