fix(clients): minor-age threshold to 18, senior/PWD doc reload, in-app document preview dialog

- Raise isMinor threshold from age <10 to <18 (legal definition), cascading correctly to
  guardian requirement, KYC document filtering, and Education Level/Degree Qualification
  visibility (now shown for all ages, not hidden for minors).
- Load and display previously-uploaded Senior Citizen proof / PWD certificate documents on
  edit (view/replace/remove), instead of always showing an empty upload prompt.
- Fix seniorPwdDocsComplete to accept an existing certificate, not just a freshly-picked file.
- Add DocumentPreviewDialogComponent: a reusable in-app modal (image/PDF/download) used in
  place of window.open() for saved KYC/address/senior-citizen/PWD documents — avoids popup
  blockers (fired only after the async download resolved) and Chrome/Edge's failure to paint
  large base64 data: URLs on first open.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/clients/client-stepper/client-preview-step/client-preview-step.component.html

@@ -195,7 +195,7 @@
                   client.isMinor ? 'preview-badge preview-badge--danger' : 'preview-badge preview-badge--verified'
                 "
               >
-                {{ client.isMinor ? 'Yes (Age < 10)' : 'No' }}
+                {{ client.isMinor ? 'Yes (Age < 18)' : 'No' }}
               </span>
             </span>
           </div>