[develop] Fix: Small fix corrections across multiple pages (1)

[qiutongMS]

Summary

• fix broken Learn API links and markup issues across multiple develop doc pages
• correct sample/API references for picker, power, spring animation, launch settings, and system button docs
• clean up small XAML and prose inaccuracies in control/layout articles

Validation

• git diff --check
• verified updated Learn API links with web fetch

[qiutongMS]

Summary: I found 2 must-fix documentation issues before this PR is ready.

Must-fix issues:

1. hub/apps/develop/files/pickers-save-file.md — section "Important APIs" / "Save a document with FileSavePicker": the page still says the picker workflow uses and returns a StorageFile, but the Windows App SDK API actually returns PickFileResult. That leaves the page internally contradictory right after the diff changed step 2 to PickFileResult.
2. hub/apps/windows-app-sdk/applifecycle/applifecycle-power.md — section "Subscribe and respond to events" (C#): the sample still declares EventRegistrationToken fields and uses them in normal C# event subscription code. The EventRegistrationToken docs say this type is an implementation detail for C# event syntax, so this sample is not valid guidance for C# readers.

[qiutongMS]

Summary: I found two blocking issues in the updated docs. Most link and terminology fixes look good, but two C++/WinRT code samples are still not valid as written.

Must-fix issues:

1. hub/apps/design/accessibility/system-button-narration.md — In the "Examples" C++/WinRT snippet, _controller is declared as winrt::SystemButtonEventController but the sample still registers events with -> (for example _controller->SystemFunctionButtonPressed(...)). In C++/WinRT this projected object is used with ./property-call syntax, so the sample does not compile as written.
2. hub/apps/develop/files/pickers-save-file.md — In the C++/WinRT FileSavePicker sample, savePicker.SuggestedFolder = L"C:\\MyFiles"; is not valid C++/WinRT setter syntax. The sample should use the projected setter form instead; otherwise readers copying the code will hit a compile error.

[qiutongMS]

Summary: I reviewed the diff and read each modified page in full. The fixes look technically correct, and the updated Learn/API links I checked resolve successfully. I did not find any must-fix issues.

Suggestion:

• hub/apps/develop/ui/controls/title-bar.md still has a front-matter label typo (label: Tile bar). Consider correcting it to Title bar in a follow-up.