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/interact/login.go

@@ -41,7 +41,7 @@ func CreateLogin() error {
 			}
 			_, err := url.Parse(s)
 			if err != nil {
-				return fmt.Errorf("Invalid URL: %v", err)
+				return fmt.Errorf("invalid URL: %v", err)
 			}
 			return nil
 		}).
@@ -69,7 +69,7 @@ func CreateLogin() error {
 		}
 		for _, login := range logins {
 			if login.Name == name {
-				return fmt.Errorf("Login with name '%s' already exists", name)
+				return fmt.Errorf("login with name '%s' already exists", name)
 			}
 		}
 		return nil
@@ -154,7 +154,7 @@ func CreateLogin() error {
 				Value(&tokenScopes).
 				Validate(func(s []string) error {
 					if len(s) == 0 {
-						return errors.New("At least one scope is required")
+						return errors.New("at least one scope is required")
 					}
 					return nil
 				}).

modules/interact/pull_merge.go

@@ -58,7 +58,7 @@ func getPullIndex(ctx *context.TeaContext, branch string) (int64, error) {
 			return 0, err
 		}
 		if len(prs) == 0 {
-			return 0, fmt.Errorf("No open PRs found")
+			return 0, fmt.Errorf("no open PRs found")
 		}
 		opts.ListOptions.Page++
 		prOptions := make([]string, 0)