feat(frontend): make stream creation validation and quote preview deterministic

[arisu6804]

Problem

Users can sign a stream with stale balance, invalid schedule, or a preview that differs from the serialized payload.

Objective

Deliver a production-quality improvement to stream creation form and preview that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Validate participant, amount, duration, start, and rate using canonical units; bind preview to exact payload and network; invalidate on changes.

Acceptance criteria

• Invalid schedules cannot reach signing; preview equals the submitted payload; stale preview requires recomputation.

Required validation

• Boundary, payload-hash, network-switch, stale-state, and component integration tests.
• Existing tests and CI remain passing.
• Add regression coverage for the original failure mode.
• Do not weaken, delete, or skip unrelated tests to obtain a green build.

PR quality bar

• Keep the PR focused and explain design tradeoffs, compatibility impact, and test evidence.
• Avoid typo-only, documentation-only, cosmetic-only, or unrelated refactor submissions.

Out of scope

• Broad rewrites not required by the acceptance criteria.
• Changes to unrelated services, contracts, or user flows.