feat(brand): use the supplied logo artwork instead of a reconstruction

The mark shipped so far was my SVG approximation, and it was wrong: the real
mark is a teardrop pin with a white window and a path flowing out of its base,
carrying three leaves — not a circle with a separate tail.

Both colourways are extracted from the supplied sheet, which has a genuinely
transparent background, so these are the artwork rather than a trace:

  public/brand/mark.png          teal pin, white window and path, green leaves
  public/brand/mark-on-dark.png  white pin with the counter knocked through

Two colourways are needed, not one. Rendered against every real ground, the
teal pin holds up on Warm White, white cards, Sand and both dark-theme grounds
— but it vanishes on the teal footer band, where only the white window
survives. The header therefore serves the teal artwork and swaps to the
on-dark artwork for dark-theme viewers through a <picture> source, needing no
JavaScript; the footer forces on-dark, because its band is teal in both
themes.

Everything downstream now derives from those two files: the favicon
(app/icon.png, replacing icon.svg), the touch icon (app/apple-icon.png,
replacing the generated apple-icon.tsx), the three PWA rasters, and the share
card, which reads public/brand/mark.png off disk so it can never drift from
the header. The logo sizing CSS keyed off a square box, which would have
squashed a 153:189 artwork — height now drives and width follows.

The wordmark stays live text in Manrope. The sheet's wordmark is a raster with
visible edge fringing, and the written style guide specifies Manrope; live
text also stays selectable, scales cleanly and recolours with the theme.

KNOWN LIMITATION: the largest instance on the sheet is 153×189. That is ample
for the header at 38px, the favicon and the share card, but short of the 512px
PWA icon, which is upscaled and slightly soft. A vector would fix it and is a
single swap — every consumer goes through components/Logo.tsx or public/brand.

Verified: tsc clean, 159/159 tests, build green, /icon.png and /apple-icon.png
emit as static routes, and the header, footer, share card and all five icons
were rendered and inspected in both themes.

nextjs-app/components/Navigation.tsx

@@ -102,14 +102,13 @@ export function Navigation() {
         <div className={styles.container}>
           <Link href="/" className={styles.logo} aria-label="schoolcompare home">
             {/*
-              LogoMark's defaults are already correct for this ground: the pin
-              takes var(--brand) and the leaf is knocked out in var(--bg-card),
-              which is exactly the header's own background in both themes. No
-              explicit pin/leaf needed here — the footer, which sits on the
-              sunken teal band, does have to pass them.
+              variant="auto": the teal pin on the white header, swapping to the
+              on-dark artwork for dark-theme viewers. The footer has to force
+              onDark instead, because its band is teal in both themes and the
+              teal pin has no silhouette against it.
             */}
             <span className={styles.logoIcon}>
-              <LogoMark />
+              <LogoMark size={38} />
             </span>
             <span className={styles.logoText}>
               school<span className={styles.logoTextAccent}>compare</span>