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/releases/utils.go

@@ -0,0 +1,29 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package releases
+
+import (
+	"fmt"
+
+	"code.gitea.io/sdk/gitea"
+)
+
+// GetReleaseByTag finds a release by its tag name.
+func GetReleaseByTag(owner, repo, tag string, client *gitea.Client) (*gitea.Release, error) {
+	rl, _, err := client.ListReleases(owner, repo, gitea.ListReleasesOptions{
+		ListOptions: gitea.ListOptions{Page: -1},
+	})
+	if err != nil {
+		return nil, err
+	}
+	if len(rl) == 0 {
+		return nil, fmt.Errorf("repo does not have any release")
+	}
+	for _, r := range rl {
+		if r.TagName == tag {
+			return r, nil
+		}
+	}
+	return nil, fmt.Errorf("release tag does not exist")
+}