Editor improvements

[NunonuN]

• Display the header level (h1, h2, h3, etc.) before headers, as Logseq and Zettlr do (I know you can see them in the Outline bar).
• Make headers foldable, with a small arrow indicator and the possibility to fold/unfold them using either a keyboard shortcut or by clicking, like Obsidian, Logseq, and Zettlr do.
• Add a keyboard shortcut to quickly switch between the two most recently used notes.
• Add support for snippets and text replacements. For example, typing -> automatically creates →.
• Add dictionaries and spell checking support.
• Add the possibility to define the tab size in the editor.
• Remember the cursor position when switching between editor mode and preview mode.
• Increment equation numbering automatically when a new $$\begin{equation}\end{equation}$$ environment is created.

[adibhanna]

Implemented on the v2.21.0 branch in commit 24c6bbe.

Included:

• Optional H1 through H6 labels before Markdown headings.
• Heading fold arrows with click controls, configurable fold and unfold shortcuts, and the existing Vim fold commands.
• Ctrl+Tab to alternate between the two most recently used notes.
• A dedicated Text replacements settings tab, enabled with -> to → by default and supporting custom rules.
• Configurable editor tab width from 1 through 8 spaces.
• Cursor position preservation when switching between Edit and Preview.
• Document-level equation numbering for unstarred LaTeX equation environments in both the editor and Preview.
• Portable config.toml support for the new settings.

Dictionaries and spell checking were intentionally left out of this work.

Validation completed in the real Electron app with isolated vault, user-data, and config directories. The run covered heading labels, click and keyboard folding, text replacement, tab width, equation numbering in Edit and Preview, cursor restoration, recent-note switching, and the settings UI with no renderer errors. Full type checking, app-core tests, shared-domain tests, desktop tests, the production Electron build, and packaged CLI verification also passed.