Expose toolbar button/icon sizing via CSS custom properties

## Problem

Toolbar buttons and icons in `Button.svelte` and `StatusBar.svelte` are hardcoded to fixed pixel values (16px button, 14px icon, 20px status bar height, 12px font, 4px gap). Consumers who need different sizing are forced to use global CSS overrides.

## Solution

Expose CSS custom properties (e.g. `--ea-toolbar-scale`) that let consumers scale the toolbar without hacks.

Start with Button.svelte and StatusBar.svelte, locating the hardcoded button, icon, status-bar, font, and gap values listed in the issue. Define CSS custom properties that let consumers scale these dimensions without global overrides; done means the toolbar sizing is configurable through those properties.

Written by the indexing model from the issue text.