Centralize and use global spacing variables

This commit improves UI consistency. It also improves maintainability by
removing "magic values" in favor of standardized spacing throughout the
application.

- Adjust spacing variables to match the convention.
- Add `_spacing.scss` to define a centralized set of spacing variables, both
  absolute and relative, to standardize the spacing throughout the application.
  This new approach ensures a consistent spacing logic across all components and
  layouts, facilitating easier maintenance and scalability of the styling codebase.
- Update various SCSS styles to utilize the new spacing variables. This change
  harmonizes the spacing across different parts of the application, aligning with
  the new design system's principles.
- Slightly adjust existing padding/margin/gaps for better consistency.

Other supporting changes per component:

- RatingCircle: Update style names to match convention and simplify
  hacky way to inject circle width value through CSS variables. Add
  tests for the new behavior and refactor existing tests for easier
  extensibility.
- TheFooter: Add small gap when footer items wrap.
- HiearchicalTreeNode: Refactor variables to separate caret size clearly
  from padding applied.
- App: Make padding responsive as initial behavior of v0.13.0 before
  5d940b57ef.
- ModalDialog: Use responsive absolute values instead of percentage.
- HorizontalResizeSlider:
  - Use `v-bind` instead of hacky way to inject SCSS values through variables.
  - Remove `verticalMargin` property to simplify its styling.
- Move `src/presentation/assets/styles/components/_card.scss` closer to
  components that it styles. Update structure documentation.

The centralization of spacing definitions will aid in future design
adjustments, ensuring that updates to spacing can be made swiftly and
uniformly across the application. It's a step towards a more maintainable
and scalable frontend architecture.

docs/presentation.md

@@ -14,14 +14,13 @@ The presentation layer uses an event-driven architecture for bidirectional react
   - [**`main.ts`**](./../src/presentation/main.ts): Starts Vue app.
   - [**`index.html`**](./../src/presentation/index.html): The `index.html` entry file, located at the root of the project as required by Vite
   - [**`bootstrapping/`**](./../src/presentation/bootstrapping/): Registers Vue components and plugins.
-  - [**`components/`**](./../src/presentation/components/): Contains Vue components and helpers.
+  - [**`components/`**](./../src/presentation/components/): Contains Vue components, helpers and styles coupled to Vue components.
     - [**`Shared/`**](./../src/presentation/components/Shared): Contains shared Vue components and helpers.
       - [**`Hooks`**](../src/presentation/components/Shared/Hooks): Hooks used by components through [dependency injection](#dependency-injections).
   - [**`/public/`**](../src/presentation/public/): Contains static assets.
   - [**`assets/`**](./../src/presentation/assets/styles/): Contains assets processed by Vite.
     - [**`fonts/`**](./../src/presentation/assets/fonts/): Contains fonts.
     - [**`styles/`**](./../src/presentation/assets/styles/): Contains shared styles.
-      - [**`components/`**](./../src/presentation/assets/styles/components): Contains styles coupled to Vue components.
       - [**`main.scss`**](./../src/presentation/assets/styles/main.scss): Main Sass file, imported by other components as single entrypoint..
   - [**`electron/`**](./../src/presentation/electron/): Contains Electron code.
     - [`/main/` **`index.ts`**](./../src/presentation/main.ts): Main entry for Electron, managing application windows and lifecycle events.
@@ -38,6 +37,13 @@ The presentation layer uses an event-driven architecture for bidirectional react
   They should also have different visual state when hovering/touching on them that indicates that they are being clicked, which helps with accessibility.
 - **Borders**:
   privacy.sexy prefers sharper edges in its design language.
+- **Fonts**:
+  - Use the primary font for regular text and monospace font for code or specific data.
+  - Use cursive and logo fonts solely for branding.
+  - Refer to [standardized font size variables](../src/presentation/assets/styles/_typography.scss) for font sizing, avoiding arbitrary `px`, `em`, `rem`, or percentage values.
+- **Spacing**:
+  Use [global spacing variables](../src/presentation/assets/styles/_spacing.scss) for consistent margin, padding, and gap definitions.
+  This provides uniform spatial distribution and alignment of elements, enhancing visual harmony and making the UI more scalable and maintainable.
 
 ## Application data