fix(fusion_claims): OWL wizard review fixes (statement handler, scss borders, tech guard)

- Move the call-type select handler into onCallType() — OWL cannot compile a
  multi-statement inline t-on body (was a render-breaking crash on mount).
- Replace color-mix() inside border shorthands with var(--sb-border) (Odoo-19
  SCSS drops color-mix in a border shorthand).
- Technician placeholder option value '' (not 'false') so the required-tech
  guard isn't bypassed.
- Remove dead setTiming(); null-coalesce the refdata onWillStart load.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

fusion_claims/static/src/xml/service_booking.xml

@@ -82,7 +82,7 @@
                                 <div class="row" t-if="!state.inShop">
                                     <label class="fl">Service call type</label>
                                     <select class="f"
-                                            t-on-change="(ev) => { const r = state.calloutRates.find(x => x.code === ev.target.value); if (r) { state.category = r.category; state.timing = r.timing; } }">
+                                            t-on-change="onCallType">
                                         <t t-foreach="state.calloutRates" t-as="r" t-key="r.code">
                                             <option t-att-value="r.code"
                                                     t-att-selected="state.category === r.category and state.timing === r.timing">
@@ -143,7 +143,7 @@
                                 <div class="row">
                                     <label class="fl">Technician</label>
                                     <select class="f" t-model.number="state.technicianId">
-                                        <option value="false">— Choose —</option>
+                                        <option value="">— Choose —</option>
                                         <t t-foreach="state.technicians" t-as="t" t-key="t.id">
                                             <option t-att-value="t.id"><t t-esc="t.name"/></option>
                                         </t>