feat: discover Firebase Emulator from FIRESTORE_EMULATOR_HOST env vars

[dan-nyanko]

Summary

FireStudio now discovers a locally running emulator via the standard Emulator Suite environment variables (FIRESTORE_EMULATOR_HOST, FIREBASE_AUTH_EMULATOR_HOST, FIREBASE_STORAGE_EMULATOR_HOST), in addition to the existing hub-locator-file scan. This makes discovery work even when the Emulator Hub (port 4400) is not running — e.g. firebase emulators:start --only firestore, or the gcloud Firestore emulator.

It also derives the project ID from FIRESTORE_EMULATOR_PROJECT / GCLOUD_PROJECT, and normalizes localhost (and wildcard bind addresses) to 127.0.0.1 so clients connect reliably over IPv4.

Problem

Previously, emulator discovery only read the hub locator file hub-<projectId>.json from os.tmpdir(), which is only written when the Emulator Hub is up. Consequences:

• When a user starts just Firestore (--only firestore, or the gcloud Firestore emulator), the hub file is absent and FIRESTORE_EMULATOR_HOST was never picked up — FireStudio reported no running emulator.
• The discovered project ID fell back to the generic demo-project, so even when connected, requests targeted the wrong project and returned no data.
• Hosts like localhost resolve to IPv6 ::1 for gRPC clients and can hang/fail to connect; only 127.0.0.1 connects reliably.

Fix

In electron/controllers/emulatorController.js:

• scanEnv() — discovers emulators from the standard env vars (authoritative signal of a running emulator) and maps them to a normalized emulator descriptor.
• parseHostPort() / normalizeHost() — parses host:port and rewrites localhost and wildcard bind addresses (0.0.0.0, ::, [::], ::1) to 127.0.0.1, since clients can't reliably connect to a wildcard/::1 address as a destination.
• readProjectIdGuess() — best-effort project ID: checks FIRESTORE_EMULATOR_PROJECT / GCLOUD_PROJECT, then .firebaserc (walking up from cwd), falling back to Firebase's conventional demo-project placeholder.
• scanRunningEmulators() — merges the hub-file scan and scanEnv(), de-duplicating by Firestore host:port so one running emulator isn't reported twice. Exported for reuse.
• The emulators:scanHub IPC handler now calls scanRunningEmulators().

Testing / verification

• Confirmed a Firestore-only emulator (FIRESTORE_EMULATOR_HOST / FIRESTORE_EMULATOR_PROJECT) is discovered with the correct project ID and connects over 127.0.0.1, and its collections appear in FireStudio.
• pnpm run lint, pnpm run format:check, pnpm run typecheck → all pass (controller is plain JS; formatted by prettier).

Notes

• Env-var discovery is additive — the existing hub-file scan still runs, and results are merged.
• This branch also includes the localStorage test-shim fix (from fix/tests-localstorage-polyfill) so CI tests pass on runtimes where jsdom doesn't expose localStorage.