chore: migrate repository URLs from AnsoCode/Sencho to studio-saelix/sencho

Updates all hardcoded GitHub repository references across 21 files:
- package.json: repository URL, bugs URL, homepage, description, author
- CONTRIBUTING.md: bug report template URL
- SECURITY.md: advisory URL, cosign cert-identity regexp
- .github/CODEOWNERS: @AnsoCode -> @studio-saelix/maintainers
- .github/workflows/ci.yml: repositories scope (Sencho -> sencho), docs-sync git URL
- .github/workflows/cla.yml: path-to-document URL
- .github/workflows/docker-publish.yml: cosign verify comment
- frontend/**/*.tsx: issues and changelog links (3 components)
- frontend/public/.well-known/security.txt: Contact and Policy URLs
- security/vex/sencho.openvex.json: @id field
- docs/openapi.yaml: license URL
- docs/docs.json: navbar and footer GitHub links (5 instances)
- docs/security.mdx: advisory and SECURITY.md links
- docs/reference/verifying-images.mdx: repo link + cosign regexp + legacy identity note
- docs/reference/contact.mdx: issues, LICENSE, advisory, policy, CoC links
- docs/reference/security-advisories.mdx: releases link
- docs/operations/verifying-images.mdx: cosign regexps and VEX download URL (6 instances)
- docs/operations/upgrade.mdx: releases links (2 instances)
- backend/src/utils/version-check.ts: GitHub Releases API endpoint

CHANGELOG.md intentionally excluded (release-please managed).
Legacy cosign identity note added for pre-migration image verification.

frontend/src/components/UserProfileDropdown.tsx

@@ -151,7 +151,7 @@ export function UserProfileDropdown({ theme, setTheme, onOpenSettings }: UserPro
                     <MenuRow
                         icon={MessageSquare}
                         label="Feedback"
-                        href="https://github.com/AnsoCode/Sencho/issues"
+                        href="https://github.com/studio-saelix/sencho/issues"
                         external
                     />
                 </div>

frontend/src/components/settings/AboutSection.tsx

@@ -32,7 +32,7 @@ export function AboutSection() {
                 <h4 className="text-sm font-medium">Links</h4>
                 <div className="flex flex-col gap-1.5">
                     <a
-                        href="https://github.com/AnsoCode/Sencho/blob/main/CHANGELOG.md"
+                        href="https://github.com/studio-saelix/sencho/blob/main/CHANGELOG.md"
                         target="_blank"
                         rel="noopener noreferrer"
                         className="text-sm text-muted-foreground hover:text-foreground transition-colors"

frontend/src/components/settings/SupportSection.tsx

@@ -23,7 +23,7 @@ export function SupportSection() {
                         </div>
                         <ExternalLink className="w-4 h-4 text-muted-foreground shrink-0" />
                     </a>
-                    <a href="https://github.com/AnsoCode/Sencho/issues" target="_blank" rel="noopener noreferrer"
+                    <a href="https://github.com/studio-saelix/sencho/issues" target="_blank" rel="noopener noreferrer"
                        className="flex items-center gap-3 p-3 rounded-lg border border-glass-border hover:bg-muted/50 transition-colors">
                         <div className="w-9 h-9 rounded-lg bg-muted flex items-center justify-center shrink-0">
                             <Bug className="w-4 h-4" />