feat(fusion_helpdesk): Critical flag, KPI cards, colored stage pills

Three coordinated changes on top of the section grouping:

1. **Mark as Critical** — a red chip on the New tab sets priority='3'
   when submitted. The central post-create hook auto-applies a "Critical"
   helpdesk.tag (shipped via fusion_helpdesk_central data XML, noupdate=1
   so support can recolor without losing it on upgrade), giving support
   a kanban-groupable signal that doesn't rely on remembering what
   priority='3' means. Scoped to in-app-channel tickets only, so a
   support agent manually setting Urgent on their own ticket isn't
   silently tagged.

2. **KPI cards above the sections** — Total / Open / Closed / Critical
   in a 4-up grid (auto-collapses to 2x2 under 540px). Each card uses
   its own saturated gradient so it reads on both light and dark mode —
   the dialog backdrop is irrelevant because the gradient brings its
   own background. Counts are computed in JS from state.tickets so they
   always match what's rendered below.

3. **Colored stage pills** — red Critical, green Solved, dark-yellow New,
   orange Cancelled, blue for In Progress / Testing / On Hold. Critical
   priority gets a *separate* red pill alongside the stage pill so you
   keep stage info even on escalated tickets. Stage matching is
   substring-based (lowercased) so a renamed "Resolved" or "Done" stage
   on central still maps to the green pill.

Tests cover the new is_critical=True → priority='3' wiring and the
default omission so SLA / stage defaults keep working for normal
tickets. Bumps fusion_helpdesk to 19.0.1.7.0 and
fusion_helpdesk_central to 19.0.1.2.0. End-to-end smoke test verified
live: priority=3 + x_fc_client_label triggers the Critical tag.

fusion_helpdesk_central/data/helpdesk_tag_critical.xml

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+    Copyright 2026 Nexa Systems Inc.
+    License OPL-1
+-->
+<odoo>
+    <!--
+        Critical tag for in-app helpdesk tickets. The client-side dialog
+        offers a "Mark as Critical" toggle on the New tab; when set, the
+        ticket is created with priority='3' (Urgent), and the central
+        post-create hook in helpdesk_ticket.py auto-applies this tag so
+        support can filter / kanban-group by it without remembering which
+        priority value means what. noupdate=1 protects support's color
+        choice from being reset by every -u fusion_helpdesk_central.
+        (noupdate lives on <data>, not on <record> — the RelaxNG schema
+        rejects it on records in Odoo 19.)
+    -->
+    <data noupdate="1">
+        <record id="tag_critical" model="helpdesk.tag">
+            <field name="name">Critical</field>
+            <field name="color">1</field>
+        </record>
+    </data>
+</odoo>