feat(editor): report uncaught errors to Better Stack

The editor reported nothing when a session died. Evidence lived only in-tab —
an 800-line React array behind a "Show console" button — so diagnosis meant
asking a user to paste a screenshot.

Better Stack's Error Tracking ingests the Sentry wire protocol, so this runs
the stock @sentry/browser against a Better Stack DSN. Sentry.init installs its
own window error/unhandledrejection handlers, so uncaught main-thread errors
and the wasm traps that escape emscripten's DOM event handlers are captured
with no instrumentation at the throw sites. Not their JS tag: it has no
beforeSend or fingerprint hooks, its runtime spawns workers from cross-origin
CDN hosts (this page is COEP: require-corp), and it ships session replay on by
default — which on a CAD canvas records customers' board geometry.

@sentry/browser is imported in exactly one file so the vendor stays swappable,
mirroring how lib/analytics.ts isolates Plausible.

Also replaces the terminal-signature regex with a shared, unit-tested predicate
(wasm/terminal-error.ts) used by BOTH the fatal overlay and the reporter, so
they cannot disagree. The regex was a type check written as a string match and
had three live holes: `RuntimeError` was listed but never appears IN
`.message`; Chrome's bare "unreachable" and "null function" matched nothing
(the v0.1.20 prod log is exactly those); and narrowing "table index is out of
bounds" to `\bindex out of bounds` for Firefox in 197f317 silently stopped
matching Chrome's spelling. Checking the TYPE — every trap in this family is a
WebAssembly.RuntimeError — covers all engines and ends the spelling chase; the
message patterns remain as a fallback for paths that lose the Error object,
such as a worker ErrorEvent crossing the realm boundary with error: null.
197f317's pthread-worker tap, promote() and Firefox findings are kept as-is.

Notes:
- Off unless VITE_ERRORS_DSN is set AND VITE_ALLOW_USER_OVERRIDE !== "1" (dev
  servers and every Playwright harness set the latter, and production builds
  never do), so a production DSN in a local .env still cannot report. With no
  DSN the whole SDK is const-folded out: 1,193,080 vs 1,282,463 bytes of JS.
- browserApiErrors integration removed. It wraps setTimeout/rAF/addEventListener
  in try/catch, which is exactly how KiCad-on-Emscripten drives its main loop.
- Console breadcrumbs off (collab/debug.ts's clog fires per Yjs update and would
  evict the ring before any crash); dom/fetch/navigation breadcrumbs kept.
- beforeSend redacts token/apiKey/Bearer — collab/provider.ts puts the collab
  token in the y-partyserver URL, so a connection-failure string carries a live
  credential — and guards the cascade: one wedge produced 8 errors in prod, and
  after the first terminal event the rest are dropped into cascade_count.

Verified end to end against the real EU host from a cross-origin-isolated page:
POST /api/<id>/envelope/ -> 200, and 4 terminal throws produce 1 event
(control: 1 throw, same count).

Privacy policy 9, cookie policy 6 and the licenses page are updated: Better
Stack is disclosed as an EU processor, and the licenses page now describes the
browser app's own JS dependencies, which it never did.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

web/.env.example

@@ -20,6 +20,9 @@ VITE_WASM_ASSET_BASE_URL=/wasm
 # Override the artifact source dir the dev symlink points at (default:
 # <repo>/tests/apps/kicad, populated by tests/scripts/setup-kicad-wasm.sh).
 # WASM_SRC_DIR=
+# Error tracking (Better Stack) — Sentry-format DSN; unset ⇒ nothing reported.
+# See standalone/.env.example for the full note.
+# VITE_ERRORS_DSN=
 
 # --- example backend ---
 # Absolute or relative path to a single KiCad project folder to serve.