feat(telemetry): report uncaught errors, and a screen to read them

[PhilippTheServer]

Closes #11 · completes S4 with OpenTaberna/fastapi#51 and OpenTaberna/frontend#5

Two halves: the admin UI now reports its own uncaught errors, and a new Errors screen
shows grouped reports from both applications — the storefront's included, because that
is where an administrator looks and where a fault costs a sale.

The screen distinguishes three states that look alike

State	What it shows
Nothing broken	"No errors reported in this period"
Nothing collected	Names FRONTEND_ERRORS_ENABLED and how to turn it on
Nothing loaded	Says the load failed

All three render an empty list if you are careless about it. Only the second is actionable
configuration, and only the third means something is wrong with the admin UI itself.

"Active in the last hour" is separate from the total. A large count with nothing recent
is a bug that has already been fixed, and it should not look like an emergency.

The reporter

Same guards as the storefront's, for the same reason — a component throwing during render
calls the handler as fast as the browser can loop:

• identical errors reported at most 3 times per session
• hard 50-per-session ceiling
• failing endpoint swallowed; console still receives the error
• nothing assumes an Error shape, since anything can be thrown

It is a copy, not a shared package. Two Angular apps in separate repositories would need
a published library to share ~150 lines, and the versioning cost outweighs the duplication.
That decision is written in the file so the next person knows a change belongs in both.

One real difference from the storefront's: the endpoint is absolute. The admin UI is served
by the Angular dev server while the API lives on another origin, so a relative path would
post to the wrong place.

Verification

$ npx ng test --watch=false
 Test Files  10 passed (10)
      Tests  67 passed (67)

$ npx prettier --check "src/app/**/*.{ts,html}"
All matched files use Prettier code style!

$ npx ng build
Initial total | 348.75 kB | 94.18 kB

Eight new tests on the screen, pinning: only still-happening errors count as active; a busy
recent error is dangerous while an old one is neutral; storefront errors are counted
separately; disabled is not confused with empty; empty is not reported as disabled; a failed
load is flagged rather than hidden; and one group expands at a time.

nav.model.spec.ts goes from seven entries to eight — an intentional update to an existing
assertion.