fix: white favicon, sidebar active-state bug, app-wide pointer cursor

Three UI issues reported after the neutral-gray redesign:

- favicon.svg was still filled #58a6ff (the pre-redesign accent blue);
  changed to white to match the sidebar logo mark.
- Sidebar nav items all showed a filled background even when inactive.
  Root cause: sidebar-menu-button.svelte (and -sub-button) rendered
  `data-active="false"` as a literal attribute, but Tailwind's bare
  `data-active:` variant matches attribute *presence*, not value — so
  data-active:bg-sidebar-accent applied to every item regardless of
  state. Fixed by emitting the attribute only when active
  (`isActive || undefined`), a latent bug in the vendored shadcn
  component that read as intentional until flagged.
- Tailwind's preflight resets <button> to cursor: default, so no button
  in the app showed a pointer. Added one base rule restoring
  cursor: pointer for buttons, [role=button], links, summary, and
  select (respecting :disabled / aria-disabled) rather than annotating
  each call site — covers new interactive elements automatically.

Risk: reversible_low (UI-only).

Verification: verified in the browser preview that inactive sidebar
items are transparent (only the current page shows a background),
nav buttons report cursor: pointer via computed styles, and the
favicon renders white in the tab.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/app.css

@@ -107,6 +107,16 @@
     line-height: 1.4;
     -webkit-font-smoothing: antialiased;
   }
+  /* Tailwind's preflight resets <button> to cursor: default; every button
+     and native interactive element in this app is clickable, so restore
+     the pointer cursor app-wide instead of annotating each one. */
+  button:not(:disabled),
+  [role='button']:not([aria-disabled='true']),
+  a[href],
+  summary,
+  select {
+    cursor: pointer;
+  }
 }
 
 #app {