Add setUser helper to core (fixes broken user identification in node + electron)

[DriesH]

Summary

Adds a unified setUser() helper to @flareapp/core, inherited by every SDK, that emits the flat attribute keys the Flare backend actually reads. This also fixes a shipped bug: @flareapp/node and @flareapp/electron were emitting enduser.* keys, which the backend never reads, so users set via those SDKs were silently never identified.

Backend reads only user.id, user.email, user.full_name, user.attributes, client.address (verified in the flareapp.io repo: RawReport.php, the User.tsx context renderer, and the protocol doc). Nothing reads enduser.*.

What changed

• @flareapp/core: new Flare.setUser(user: User | null): this. Maps id→user.id (stringified), email→user.email, fullName→user.full_name, ipAddress→client.address; extra keys bundle into user.attributes; setUser(null) clears. Sugar over the active scope's pendingAttributes, so it is scope-aware for free (browser/electron single scope, node per-request). Exposes USER_IDENTITY_KEYS + userIdentityAttributes(scope) as the single source of truth.
• @flareapp/node: removed the divergent setUser override, NodeScope.user, the provider setUser, the enduser.* collector block, and the local User type. Inherits core setUser; User re-exported from core.
• @flareapp/electron: removed the setUser override, private user field, projectUser, and the ElectronUser type. Keeps a mainScope reference so the forwarded-renderer overlay can stamp main-authoritative identity (sourced from the same scope core writes to) onto reports that bypass core's pipeline.
• @flareapp/js: re-exports User.
• Docs: setUser documented across the js/node/electron/react/vue/svelte/sveltekit READMEs.

Breaking changes (pre-1.0 packages, intentional)

• @flareapp/node: setUser's username field removed (use fullName, or pass as an extra key → user.attributes).
• @flareapp/electron: ElectronUser type removed (User from core); username likewise gone.
• Both: emitted keys change enduser.* → user.*. This is the fix.

Follow-ups (out of scope)

• Flare docs site (separate flareapp.io repo): a "Data collection > Identifying users" page for the JS SDKs. Handoff instructions prepared at docs/handoffs/2026-06-23-flare-docs-identifying-users.md.
• Deferred Playwright e2e exercising setUser through the real fetch path (unit tests already assert the sent report payload).

Test Plan

• npm run typescript clean repo-wide
• npm run build clean (all packages)
• npm run test green across all workspaces
• Core unit tests assert user.*/client.address/user.attributes on the sent report payload, null-clear, overwrite-drops-omitted, and the id: null guard
• Node integration test asserts user.* on the report and per-request scope isolation
• Electron test asserts main-origin user.* and that main identity overrides a renderer-supplied user.id on forwarded reports
• No enduser.* remains in shipping src