Sell the rectangle instead of the sponsored line

[ralyodio]

Switches the ad from the inline text link to the 300×250 rectangle.

The text link was chosen because it is the only format that fills its container, and so the only one safe in a column whose width nothing here can measure without a script. That reasoning holds — it is also the smallest thing the slot can sell. The rectangle is the inventory worth having, so the column gets made to fit it instead.

One size, everywhere

banner_300x250. 300px is the widest fixed creative that still fits a 320px phone, which is what makes a single size viable at all:

• picking a size by viewport would need a script, and the whole embed exists to avoid running one;
• rendering two units so CSS can hide one bills two impressions for one reader — serving meters server-side at fill time, whether or not anyone saw it.

The columns that were too narrow

Every creative is laid out at its format's exact pixel width inside the frame, so a narrow column crops it rather than reflowing it. Measured before the fix:

Viewport	Column	Result
320px, test page	250px	cropped 50px — a fifth of the ad
360px, test page	290px	cropped 10px
390px+	≥300px	fine

Below 640px the unit now escapes its column's padding and centres against the viewport (margin-inline: calc(50% - 50vw)). The ad does not get bigger; it stops being cropped by a gutter. body already hides horizontal overflow, so a stray pixel cannot start a sideways scroll.

Re-measured at 320 / 360 / 390 / 430 / 768 / 1280 on both pages: creative is 300px everywhere, cropped nowhere, and scrollWidth === clientWidth at every width including either side of the 640px breakpoint.

Theme

Still nothing asks for one, and that is now deliberate rather than a gap. Since profullstack/crawlproof.com#221, /api/ads/frame ships both palettes behind prefers-color-scheme and the frame answers it from the reader's own browser — a better signal than this end could offer, given the site's theme is a stored preference the server never sees.

Verified on the same page: rgb(7,10,16) for a reader in dark, rgb(246,249,251) for one in light. The dark bar on the light theme is gone.

Tests

The test pinning format=text_link pinned the reasoning this PR reverses, so it is replaced rather than deleted — the new one holds the format to a single fixed size and the frame to that format's exact box, which is the property that breaks if either drifts. A second new test holds the embed to naming no theme, so nobody re-adds a guess that would override the reader's browser.

Verified

biome check clean · 66 tests pass · static export still ad-free (desktop app untouched) · all 24 pages pass the browser smoke test · no CSP violations and no page errors in a real browser at desktop and mobile widths in both themes.