Add hide/show details toggle to NotePanel metadata

[multiplex55]

Motivation

• Provide a simple per-panel runtime control to hide or show metadata (Tags, Links, Backlinks) so users can focus on the note content while preserving fine-grained metadata UI state.
• Keep existing editor/preview behavior and accessibility/focus handling intact when toggling metadata visibility.

Description

• Added a new boolean field show_metadata: bool to NotePanel and initialized it to true in NotePanel::from_note so panels default to showing metadata.
• Added details_toggle_label() helper and a top-row toggle button (renders Hide Details / Show Details) placed before the font-size controls; toggling reuses the existing focus_textedit_next_frame pattern to preserve keyboard focus behavior.
• Wrapped rendering of the Tags row, Links row, and Backlinks section in ui with if self.show_metadata { ... } guards; the editor/preview content area remains always visible and backlink tabs/pagination state are preserved when hiding/showing.
• Added unit tests in the existing #[cfg(test)] module: toggle_hides_metadata_sections_in_ui, toggle_button_label_reflects_state, and toggle_preserves_tab_and_pagination_state, and a small helper render_panel_and_dump_shapes to exercise UI rendering in tests.

Testing

• Ran cargo fmt -- src/gui/note_panel.rs successfully to format the changes.
• Attempted to run targeted tests (e.g. cargo test toggle_button_label_reflects_state), but the test build failed in this environment due to a missing system dependency required by a transitive crate (alsa-sys requires a system alsa pkg-config file), preventing completion of cargo test runs here.
• The added unit tests are included in the PR and should pass in a normal development environment with standard system build dependencies installed.

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Codex Task