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/auth/oauth.go

@@ -226,7 +226,6 @@ func startLocalServerAndOpenBrowser(authURL, expectedState string, opts OAuthOpt
 	codeChan := make(chan string, 1)
 	stateChan := make(chan string, 1)
 	errChan := make(chan error, 1)
-	portChan := make(chan int, 1)
 
 	// Parse the redirect URL to get the path
 	parsedURL, err := url.Parse(opts.RedirectURL)
@@ -311,7 +310,6 @@ func startLocalServerAndOpenBrowser(authURL, expectedState string, opts OAuthOpt
 	if port == 0 {
 		addr := listener.Addr().(*net.TCPAddr)
 		port = addr.Port
-		portChan <- port
 
 		// Update redirect URL with actual port
 		parsedURL.Host = fmt.Sprintf("%s:%d", hostname, port)