refactor: code cleanup across codebase (#947)

## Summary

- Extract duplicate \`getReleaseByTag\` into shared \`cmd/releases/utils.go\`
- Replace \`log.Fatal\` calls with proper error returns in config and login commands; \`GetLoginByToken\`/\`GetLoginsByHost\`/\`GetLoginByHost\` now return errors
- Remove dead \`portChan\` channel in \`modules/auth/oauth.go\`
- Fix YAML integer detection to use \`strconv.ParseInt\` (correctly handles negatives and large ints)
- Fix \`path.go\` error handling to use \`errors.As\` + \`syscall.ENOTDIR\` instead of string comparison
- Extract repeated credential helper key into local variable in \`SetupHelper\`
- Use existing \`isRemoteDeleted()\` in \`pull_clean.go\` instead of duplicating the logic
- Fix ~30 error message casing violations to follow Go conventions
- Use \`fmt.Errorf\` consistently instead of string concatenation in \`generic.go\`

Reviewed-on: https://gitea.com/gitea/tea/pulls/947
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: Bo-Yi Wu (吳柏毅) <appleboy.tw@gmail.com>
Co-authored-by: Nicolas <bircni@icloud.com>
Co-committed-by: Nicolas <bircni@icloud.com>

cmd/login/delete.go

@@ -5,8 +5,7 @@ package login
 
 import (
 	"context"
-	"errors"
-	"log"
+	"fmt"
 
 	"code.gitea.io/tea/modules/config"
 
@@ -27,7 +26,7 @@ var CmdLoginDelete = cli.Command{
 func RunLoginDelete(_ context.Context, cmd *cli.Command) error {
 	logins, err := config.GetLogins()
 	if err != nil {
-		log.Fatal(err)
+		return err
 	}
 
 	var name string
@@ -37,7 +36,7 @@ func RunLoginDelete(_ context.Context, cmd *cli.Command) error {
 	} else if len(logins) == 1 {
 		name = logins[0].Name
 	} else {
-		return errors.New("Please specify a login name")
+		return fmt.Errorf("please specify a login name")
 	}
 
 	return config.DeleteLogin(name)