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/git/branch.go

@@ -80,7 +80,7 @@ func (r TeaRepo) TeaFindBranchBySha(sha, repoURL string) (b *git_config.Branch,
 		return nil, err
 	}
 	if remote == nil {
-		return nil, fmt.Errorf("No remote found for '%s'", repoURL)
+		return nil, fmt.Errorf("no remote found for '%s'", repoURL)
 	}
 	remoteName := remote.Config().Name
 
@@ -133,7 +133,7 @@ func (r TeaRepo) TeaFindBranchByName(branchName, repoURL string) (b *git_config.
 		return nil, err
 	}
 	if remote == nil {
-		return nil, fmt.Errorf("No remote found for '%s'", repoURL)
+		return nil, fmt.Errorf("no remote found for '%s'", repoURL)
 	}
 	remoteName := remote.Config().Name