The font-size setting resets on every page load and is not offered on mobile

[mmcky]

Found in the review of the #147 gate (study row F023). It is filed without a parent, because the text stays readable at the default size.

Problem

• The choice isn't kept. The toolbar's font-size buttons change the page's root font size, but the choice lives only in React state and an inline style. A reload, a new tab or a fresh visit returns to the default. Measured on 2026-09-11 in Chromium, on lecture-python-programming's jb2 preview: 16px by default, 17.6px after one step up, 16px again after a reload.
• There are no buttons on mobile. They are hidden md:block, and the mobile "More actions" menu has no font-size entry.

The book theme stores the choice in localStorage.toolbarFont and re-applies it on load (theme-settings.js L52-L88). Its steps are ×1.2 and ×0.9 (_base.scss L17-L64), where this theme uses 0.9rem and 1.1rem.

Where it happens.

• FontScaleListItems.tsx L6-L29: useState, then document.documentElement.style.fontSize, with nothing stored.
• Toolbar.tsx L66-L69: the buttons are desktop-only.
• MobileActionsMenu.tsx L33-L39: the mobile menu has no font-size entry.

Proposed change

• Remember the choice. Store the step in localStorage (wrapped in try/catch), and apply it early enough that the text doesn't visibly jump after load.
• Add it to the mobile menu.
• Step sizes: decide whether to match the book theme's.
• Keep it rem-based: the root stays at the browser default until a reader chooses, so the browser's own text-size setting still applies.

Acceptance criteria

• A font-size choice survives a reload and applies on other pages of the same site.
• The stored size is applied without a visible jump after the page loads.
• The control is available in the mobile "More actions" menu.
• A test covers persistence across a reload.

Context

• Source: the 2026-09-11 parity study, row F023 (recorded in PLAN.md and the user docs claim book-theme parity and behaviour that the code does not deliver #210, now closed).
• Related: A reader's light or dark choice is overwritten when the operating system switches appearance #211 covers the other stored reader preference, the light or dark choice.