Add window size and position persistence

[Copilot]

Implements window geometry persistence to registry, matching native Notepad behavior.

Implementation

• State: Added windowX, windowY, windowWidth, windowHeight to AppState with 640×480 default
• Storage: New LoadWindowSettings() / SaveWindowSettings() functions write to HKEY_CURRENT_USER\Software\LegacyNotepad
• Lifecycle: Load on startup before CreateWindowExW(), save on WM_DESTROY using GetWindowPlacement().rcNormalPosition

Edge Cases

• Multi-monitor: Negative coordinates preserved via static_cast<int>(DWORD) round-trip
• Off-screen: MonitorFromRect() validation resets to CW_USEDEFAULT if no monitor intersection
• Maximized state: Uses rcNormalPosition so window doesn't reopen maximized unintentionally

Registry keys: WindowX, WindowY, WindowWidth, WindowHeight (REG_DWORD)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Remember window size and maybe position</issue_title>
<issue_description>Hello again!

Not really a bug or an issue but the og notepad remembers the window size and position of the last session. Since you already put font and always on top settings in the registry I guess this can be put there too?

Have a nice day and thanks!</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Remember window size and maybe position #20

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