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/clone.go

@@ -76,9 +76,13 @@ func runRepoClone(ctx stdctx.Context, cmd *cli.Command) error {
 
 	owner, repo = utils.GetOwnerAndRepo(url.Path, login.User)
 	if url.Host != "" {
-		login = config.GetLoginByHost(url.Host)
+		var lookupErr error
+		login, lookupErr = config.GetLoginByHost(url.Host)
+		if lookupErr != nil {
+			return lookupErr
+		}
 		if login == nil {
-			return fmt.Errorf("No login configured matching host '%s', run `tea login add` first", url.Host)
+			return fmt.Errorf("no login configured matching host '%s', run 'tea login add' first", url.Host)
 		}
 		debug.Printf("Matched login '%s' for host '%s'", login.Name, url.Host)
 	}