Improve site text readability

[Kashkovsky]

What changed

• introduced one shared --font-size-min: 12px typography token
• raised every explicit site text size below 12px to that shared minimum
• consolidated the requested selectors into one grouped CSS rule
• kept headings and existing text sizes above the minimum unchanged
• added a regression test preventing explicit sub-12px text sizes

Why

Small labels, body copy, code, and demo text were difficult to read. This establishes a site-wide 12px floor without duplicating the value across selectors.

Validation

• bun run site:check (48 tests)
• bun run site:build
• computed-font audit across all six routes at desktop and 390px mobile widths: no visible text below 12px
• no horizontal overflow on the audited routes