Stabilize note panel window identity

[multiplex55]

Motivation

• Prevent note window remembered size/position from resetting when the Markdown-derived note title changes.
• Keep the visible window title driven by the note (self.note.title.clone()) and preserve .resizable(true) behavior.

Description

• Create a stable slug-derived window id with let window_id = egui::Id::new(("note_panel_window", self.note.slug.clone())); before building the window.
• Attach the stable id to the window builder via .id(window_id) while still calling egui::Window::new(self.note.title.clone()) and keeping .resizable(true).
• Add a unit test markdown_title_change_keeps_note_window_rect that renders the panel, mutates the note title/content, and asserts the remembered rect from ctx.memory(|m| m.area_rect(window_id)) remains unchanged.
• Run rustfmt with edition 2024 on the modified file.

Testing

• Ran rustfmt --edition 2024 src/gui/note_panel.rs which completed successfully.
• Ran cargo test markdown_title_change_keeps_note_window_rect --lib, which initially failed due to missing system pkg-config dependencies for alsa during the build.
• After installing the required native packages (libasound2-dev and X11 development libs) and re-running the test, the build failed due to unrelated platform-specific compile errors (windows::Win32 / windows::core and other OS-specific bindings) when compiling the full crate on Linux, preventing end-to-end validation of the test in this environment.

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Codex Task