Give each call its own page, and a form to answer it

A call now has its address — description, what the volunteer gets, the
commitment added up, level, closing date, project, technologies — and
the application form right there. The landing's call card leads to it
instead of scrolling to the general form, and shows the level in the
visitor's language rather than the API's `mid` or `any`.

The form is the landing's, with a `callSlug` that hides the kind
selector and posts to the new /api/calls/{slug}/apply handler: the API
refuses a `kind` on an application, which is always an offer of time.
Both handlers share one check, core/submission.ts, now tested on its
own.

A call that is filled or closed keeps its page — a link to it may
still be out there — but shows why there is no form, and where to
write instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/mock-api.mjs

@@ -133,6 +133,20 @@ for (const listed of data['/public/v1/projects']) {
   };
 }
 
+// Calls link to their own page, and one of them is closed so the page can
+// show what a visitor sees when a link to a filled call is still out there.
+for (const call of data['/public/v1/calls']) {
+  call.path = `/colabora/${call.slug}`;
+  call.alternates = { es: call.path, en: `/en/contribute/${call.slug}`, ca: `/ca/col-labora/${call.slug}` };
+  data[`/public/v1/calls/${call.slug}`] = {
+    ...call,
+    status: call.id === 2 ? 'filled' : 'open',
+    what_you_get: 'Revisiones de código con alguien del colectivo.\nUn proyecto real en tu portfolio.',
+    total_hours: call.hours_per_week * (call.duration_weeks ?? 4),
+    project: { ...call.project, path: `/proyectos/${call.project.slug}` },
+  };
+}
+
 for (const listed of data['/public/v1/members']) {
   data[`/public/v1/members/${listed.slug}`] = {
     ...listed,
@@ -150,6 +164,14 @@ createServer((req, res) => {
 
   res.setHeader('content-type', 'application/json');
 
+  // The two write endpoints: accept whatever arrives, as the real API does
+  // for a valid submission.
+  if (req.method === 'POST') {
+    res.statusCode = 201;
+    res.end(JSON.stringify({ data: { id: 1, kind: 'contribute', status: 'new' } }));
+    return;
+  }
+
   if (payload === undefined) {
     res.statusCode = 404;
     res.end(JSON.stringify({ message: 'not found' }));