buttons: one definition of the shape, worn by a <button> and by an <a>

The download links added in fd1e4ae carried their own copy of BaseButton's
class list, because BaseButton is a <button> and cannot hold an href — and a
download must be an anchor, so the browser's own download manager gets the
3-95 MB transfer instead of a blob this app would have to hold in memory.

A copy is not a solution to that; it is two primary buttons that look alike
until someone changes one. So the shape moves to `.btn` + `.btn-primary` /
`.btn-ghost` in the components layer, where both elements can wear it, and
neither owns it.

The `disabled:` variants stay on BaseButton. An anchor has no :disabled, so
they were never shared and pretending otherwise would put a rule in the
shared definition that only one of its two users can ever match.

Verified there is exactly one shape to unify and no third copy: `px-4 py-2.5`
appears in three other files and all three are something else (a toast, a
dashed quick-add affordance, a retention notice). The smaller brand buttons in
AppShell and NoteEditor are a different size, which is a size-variant question
and not this one. And exactly one call site passes a class to BaseButton —
`shrink-0` — which cannot conflict with anything the shape declares.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K3MMqUtzX1TJgA1oypvm1c

frontend/src/components/ClientDownloads.vue

@@ -143,17 +143,20 @@ function readableSize(bytes: number): string {
               >
             </p>
           </div>
-          <!-- A plain anchor, never BaseButton and never a fetch: these are 3–95 MB
-               and the browser's own download manager handles the transfer better
-               than anything this app would do with a blob. `download` carries no
-               filename because the server already names the file in its
-               Content-Disposition, which browsers prefer over this attribute
-               anyway — a value here would be inert and read as if it weren't. -->
+          <!-- An anchor, never BaseButton and never a fetch: these are 3–95 MB and
+               the browser's own download manager handles the transfer better than
+               anything this app would do with a blob. It wears `.btn` — the same
+               definition BaseButton wears, so the two cannot drift.
+
+               `download` carries no filename because the server already names the
+               file in its Content-Disposition, which browsers prefer over this
+               attribute anyway — a value here would be inert and read as if it
+               weren't. -->
           <a
             :href="client.url"
             download
-            class="btn-link shrink-0"
-            :class="group.prominent ? 'btn-link-primary' : 'btn-link-ghost'"
+            class="btn shrink-0"
+            :class="group.prominent ? 'btn-primary' : 'btn-ghost'"
           >
             Download
           </a>