fix(release): nightly 仅发布到 GitHub，不再上传 R2

[Soulter]

Summary / 改动概述

nightly 发布目前会上传更新包和更新清单到 R2。本次将 R2 上传、公开对象校验和通道清单更新限制为 stable（tag-poll）发布，nightly 仅发布到 GitHub Releases。

同时将 nightly 更新清单中的下载地址和客户端默认更新入口改回 GitHub Releases，避免停止 R2 上传后仍从旧地址检查或下载更新。stable 继续使用 R2，并同步更新文档。

Verification / 验证方式

• node --test scripts/ci/build-desktop-tauri-workflow.test.mjs scripts/prepare-resources/tauri-config.test.mjs scripts/ci/release-updater-artifacts.test.mjs：21 项通过。
• python3 -m unittest scripts.ci.test_generate_tauri_latest_json scripts.ci.test_publish_r2_release：45 项通过。
• 回归测试实际执行工作流中的 manifest 生成脚本，验证 nightly 在未配置 R2 公网地址时生成 GitHub 下载链接，stable 仍生成 R2 下载链接。
• git diff --check：通过。
• 本次提交的 CI 已触发，仍在运行：Lint Workflows、Check Scripts、Check Rust。

已有客户端若内置旧的 R2 nightly 入口，需要安装包含新入口的版本或使用更新入口环境变量覆盖；本次未迁移、删除 R2 上的历史对象。

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Checklist / 检查清单

• This change is not a breaking change. / 此改动不是破坏性变更。（旧客户端入口迁移限制见上。）
• I have verified the change locally (and provided logs/screenshots above). / 我已在本地验证并在上方提供日志/截图。
• If workflows are changed, I attached at least one related Actions run link. / 如果修改了工作流，我已附上至少一个相关 Actions 运行链接。
• If dependencies are updated, lockfiles are updated accordingly. / 未修改依赖，无需更新 lock 文件。
• This PR does not include malicious code. / 此 PR 不包含恶意代码。

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Needs a human reviewer. Nightly updater clients are redirected to GitHub Releases, while the workflow stops publishing nightly updater objects to R2. If the release URL, manifest, or asset publication is wrong, nightly users may fail to update or receive an unintended artifact; reverting restores the previous path, but already-published GitHub release artifacts and any failed update attempts are not undone.

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