feat: install @sentry/vue + observability module skeleton

WS-7 PR-3 commit 1. Frontend mirror of the backend SDK install
(commits bdb89a2..adab3be), wired against the existing apps/app SPA.

- pnpm add @sentry/vue@10.52.0 (pinned).
- src/observability/sentry.ts: initSentry() — empty DSN no-op (RFC §3.3),
  errors-only (tracesSampleRate=0, profilesSampleRate=0; RFC §2 amend.B),
  sendDefaultPii=false, Console integration off, beforeSend wired to the
  scrubber, initial scope tag app=app for GlitchTip filtering.
- src/observability/scrubber.ts: TypeScript port of backend
  SentryEventScrubber. RFC §3.7 frontend block — body / header / query
  scrubbing, form_values wholesale replacement, cookies wholesale,
  defensive strip of contexts.storage and user.cookies, max-depth guard.
- src/observability/contextBinding.ts: Vue Router beforeEach guard that
  binds RFC §3.6 auth-scope tags per navigation. Three zones via
  route.meta.public + route.path matching:
    - portal token zone (meta.public + meta.context=portal) → actor_scope=
      portal, no user_id (RFC §3.6 explicit)
    - /platform/* with super_admin → actor_scope=platform, no org tag
    - default authenticated → actor_scope=organisation when an active
      organisation is selected (useOrganisationStore.activeOrganisationId),
      otherwise actor_scope=user
    - unauthenticated public pages → actor_scope=anonymous
  Reads useAuthStore (user, appRoles, isSuperAdmin) and
  useOrganisationStore (activeOrganisationId) — corrected vs. RFC's
  speculative auth-store API.
- src/observability/index.ts: barrel.
- src/main.ts: initSentry runs before registerPlugins so Sentry's Vue
  errorHandler hooks before any plugin or component initialises;
  installContextBinding runs after registerPlugins so pinia is up.
- env.d.ts: VITE_SENTRY_DSN_FRONTEND + VITE_SENTRY_RELEASE typed.
- .env.example: new file (didn't exist before) documenting all SPA env
  vars including the new Sentry pair.
- vite.config.ts: build.sourcemap=true (RFC §3.5 — generated, uploaded
  to GlitchTip by deploy.sh, then stripped before nginx serves dist/).

Typecheck: green. Build: green, *.map files emitted alongside *.js
chunks as expected.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/app/package.json

@@ -20,6 +20,7 @@
     "@casl/ability": "6.7.3",
     "@casl/vue": "2.2.2",
     "@floating-ui/dom": "1.6.8",
+    "@sentry/vue": "10.52.0",
     "@sindresorhus/is": "7.1.0",
     "@tanstack/vue-query": "^5.95.2",
     "@tiptap/extension-highlight": "^2.27.1",