Keep streaming insertion enabled for migrated prefs

Older builds could persist the old default streamingInsert=false into preferences.json, which is indistinguishable from a user opt-out if the value is treated as an ordinary bool. Add a one-time migration marker, persist normalized legacy prefs on load, and keep later user opt-outs durable after migration.

Constraint: issue #440 requests streaming input/output defaults on for fresh installs and updates, plus release notes for the behavior change.

Rejected: treating any stored false as manual opt-out | old default writes would keep update users off forever.

Rejected: overriding every false forever | would erase a user's post-migration manual opt-out.

Confidence: high

Scope-risk: moderate

Directive: Keep UserPreferencesWire serde defaults and migration markers aligned with persisted UserPreferences fields.

Tested: cargo test --manifest-path src-tauri/Cargo.toml --lib streaming_insert -- --nocapture

Tested: cargo test --manifest-path src-tauri/Cargo.toml --lib legacy_streaming_insert_false_is_migrated_and_marker_is_persisted -- --nocapture

Tested: npm run build

Tested: cargo test --manifest-path openless-all/app/src-tauri/Cargo.toml --lib

Tested: git diff --check

Not-tested: cargo fmt --check fails on pre-existing unrelated formatting drift in volcengine.rs and commands.rs.

.github/workflows/release-tauri.yml

@@ -505,6 +505,11 @@ jobs:
             - 以 `-tauri` 结尾的 tag 是**正式版**（自动推送给所有 in-app 检查更新的用户）。
             - 以 `-beta-tauri` 结尾的 tag 是 **Beta 版**（GitHub 标 pre-release，**不**通过 in-app updater 推送给正式版用户；只对在「设置 → 关于 → 加入 Beta 渠道」开关切到 Beta 的用户可见，且需要手动从此页面下载安装）。
 
+            ### 行为变更提示
+
+            - 流式输入默认开启；不兼容场景会自动回落到一次性插入。可在「设置 → 高级」关闭。
+            - 流式输入成功后默认把最终文本同步到剪贴板，方便再次粘贴；可在「设置 → 高级」关闭。
+
           append_body: true
           # Matrix jobs all upload assets to the same release. Generate notes once
           # so macOS, Windows, and Linux jobs do not duplicate the release body.