fix(analytics): put the Buy class on the <form> so the goal actually fires

Plausible's click handler walks up from the clicked element and bails out the
moment it meets a <form>, so a plausible-event-name=Buy class on a
<button type="submit"> inside a plain form never fires Buy — the tracker sends
its automatic "Form: Submission" event instead and the goal records nothing.
Measured live on /odoo-erp-automation-bootcamp/ before this change:
{"n":"Form: Submission"}.

- odoo-erp-automation-bootcamp/index.html (3 CTAs): class moved from the
  <button type="submit"> to the enclosing <form>.
- australia.html (2 CTAs): same move.
- de|es/odoo-erp-automation-bootcamp/index.html (3 forms each): the buy forms
  carried no Buy marking at all, so the DE and ES bootcamp sales were invisible
  to the goal. Class added to the <form>.
- odoo/upsell.html: the three .plan-btn buttons drive the same JS buy path and
  were unmarked. A class on a plain <button> (not a form submit) does fire, so
  it was added there and browser-verified. The disabled Trial button is left
  unmarked.

Class attribute only: every button label, every form action and every basket
payload is byte-identical. No CSS or JS rule references the class.

Verified in a real browser (fetch shimmed, tracker's localhost/automation gates
bypassed): EN bootcamp 3/3, australia 2/2, DE 3/3, ES 3/3, upsell 1/1 clicks
each send exactly one {"n":"Buy"} to /api/event and never "Form: Submission".

odoo/upsell.html

@@ -248,7 +248,7 @@
                 <div class="plan-tagline">
                     "ODOO Standard .. that is all my business needs"
                 </div>
-                <button class="plan-btn" id="btn-sidehustle">
+                <button class="plan-btn plausible-event-name=Buy" id="btn-sidehustle">
                     Upgrade Now
                 </button>
                 <ul class="plan-features">
@@ -268,7 +268,7 @@
                 <div class="plan-tagline">
                     "I need some modules and professional reports"
                 </div>
-                <button class="plan-btn" id="btn-ontherise">Upgrade Now</button>
+                <button class="plan-btn plausible-event-name=Buy" id="btn-ontherise">Upgrade Now</button>
                 <ul class="plan-features">
                     <li>ODOO CE hosting</li>
                     <li>Space for 4 Backups</li>
@@ -286,7 +286,7 @@
                 <div class="plan-tagline">
                     "ODOO Enterprise is a must have!"
                 </div>
-                <button class="plan-btn" id="btn-powerhouse">
+                <button class="plan-btn plausible-event-name=Buy" id="btn-powerhouse">
                     Upgrade Now
                 </button>
                 <ul class="plan-features">