No global ErrorBoundary — any unhandled exception crashes the whole app with no recovery UI

[ndii-dev]

RootLayout in app/_layout.tsx wraps the Stack in nothing but a StatusBar, with no error boundary. A malformed SecureStore read, a Horizon SDK throwing an unexpected shape, or any other unhandled exception in a screen currently takes down the entire app to a native red-screen/crash with zero recovery path for the user — unacceptable for an app where the crash could occur mid-transaction-review.

Definition of done:

• Global ErrorBoundary at the root layout level with a real recovery UI (not just "app crashed", an actionable retry/restart affordance)
• Verified it does NOT swallow errors during signing/broadcast in a way that leaves the user unsure whether a transaction went through — that ambiguity is worse than a crash
• Crash is still reported (once a crash reporter exists, per the related guardrails issue) with context, just not fatal to the user session

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Before opening a PR for this issue, read CONTRIBUTING.md.

This is not a starter-issue. The Definition of done above is the full
acceptance criteria, not a subset to sample from — a PR that addresses part
of it is an unfinished issue, not a smaller one. Your PR must include, in
the PR description itself:

• Root cause / design-decision rationale in your own words — not a restatement of this issue
• Every Definition of done bullet above addressed explicitly, with a one-line note on how
• Evidence the code actually runs: pasted test/build output, a screen recording or before/after screenshots for UI changes, or real (non-mocked) logs for network/contract-facing work
• New or updated tests included and shown passing (paste the output)
• Any adjacent/related behavior this issue calls out re-verified, not assumed unaffected

PRs missing these will be sent back before review, not reviewed and rejected — please do this up front.