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>

modules/utils/validate.go

@@ -21,17 +21,17 @@ func ValidateAuthenticationMethod(
 	// Normalize URL
 	serverURL, err := NormalizeURL(giteaURL)
 	if err != nil {
-		return nil, fmt.Errorf("Unable to parse URL: %s", err)
+		return nil, fmt.Errorf("unable to parse URL: %s", err)
 	}
 
 	if !sshAgent && sshCertPrincipal == "" && sshKey == "" {
 		// .. if we have enough information to authenticate
 		if len(token) == 0 && (len(user)+len(passwd)) == 0 {
-			return nil, fmt.Errorf("No token set")
+			return nil, fmt.Errorf("no token set")
 		} else if len(user) != 0 && len(passwd) == 0 {
-			return nil, fmt.Errorf("No password set")
+			return nil, fmt.Errorf("no password set")
 		} else if len(user) == 0 && len(passwd) != 0 {
-			return nil, fmt.Errorf("No user set")
+			return nil, fmt.Errorf("no user set")
 		}
 	}
 	return serverURL, nil