Replace placeholder logo with user's whale artwork

- logo.png: user's whale (1024px JPEG) with background flood-filled to
  transparent at fuzz 3 (preserves white belly details), alpha
  thresholded to kill JPEG-noise halos, trimmed and centered on
  720px canvas. v3 verified clean: no streaks, no box lines, no fringe.
- favicon.png (32px) and apple-touch-icon.png (180px) regenerated
  from the same source.
- embed.png (1200x630) rebuilt: whale at 440px left, no overlap with
  the WhalePrep title text.
- views.rs: nav brand now references logo.png instead of logo.svg.

src/views.rs

@@ -133,7 +133,7 @@ fn public_origin() -> String {
 
 fn nav(ctx: &Ctx, user: Option<&str>) -> String {
     let brand = format!(
-        r##"<header class="topbar"><div class="brand-group"><a class="brand" href="{b}/"><img class="brand-logo" src="{b}/static/logo.svg" alt="WhalePrep logo" width="28" height="28"> WhalePrep</a></div><nav class="nav">"##,
+        r##"<header class="topbar"><div class="brand-group"><a class="brand" href="{b}/"><img class="brand-logo" src="{b}/static/logo.png" alt="WhalePrep logo" width="28" height="28"> WhalePrep</a></div><nav class="nav">"##,
         b = ctx.base
     );
     let links = match user {