fix(css): restore sans-serif UI font

[rdmiller]

Several parts of the UI render in a serif face: flexlayout tab labels, "No Results Yet!" in the results panel, and the Heap/FPS readout over the canvas.

Cause

src/css/styles.css carried a universal rule with no fallback, depending on a webfont pulled from Google Fonts:

@import url(https://fonts.googleapis.com/css?family=Lato:...);
* { font-family: "Lato"; }

Host apps block that request under their CSP, so Lato never loaded — and with no fallback in the declaration, every element the rule matched dropped to the browser's default serif. MUI components were unaffected because their emotion classes outrank * and carry the theme's own sans stack, which is why the app looked half-right rather than obviously broken.

Evidence

Measured in the running app rather than inferred:

• document.styleSheets contained zero Google Fonts entries — the @import never landed.
• Text set in Lato measured 327.02px; the same text in a deliberately nonexistent family __NoSuchFont__ measured 327.02px. Identical, so Lato was resolving to nothing. Arial measured 366.38px for comparison.

Fix

Introduces --cram-font-family holding the same stack the MUI theme uses, applies it to * and .App, and drops the now-unused external font request.

Using the MUI stack rather than a bare Arial means unstyled elements resolve to the same font Typography already uses, instead of introducing a second sans face. Arial remains in the stack as the conventional fallback.

workbenchTheme.css feeds the same variable into flexlayout's --font-family, keeping a literal fallback in case those styles are ever consumed without styles.css.

Verification

Confirmed in the browser against project-varese — flexlayout root, tab buttons, "No Results Yet!" and the Heap/FPS label all now compute to the sans stack, and render accordingly.

Also swept for other fallback-less font-family declarations in CSS and inline fontFamily in components: there are none, so these were the only affected paths.

2226 tests / 104 files pass, lint and typecheck clean. CSS-only change; no behavioural surface.

Split out of #87, which is unrelated feature work. No file overlap between the two.

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