fix(web): validate vesting timestamp ordering before submission

[Anti83]

The web form currently accepts schedules that VestingSchedule::validate rejects on-chain. For example, Linear start=200/end=100 and CliffLinear start=100/cliff=201/end=200 both return ok: true from buildVestingSchedule, so users only learn about the invalid dates when the transaction fails.

Align the form builder with the program's existing rules:

• Linear and CliffLinear: end must be strictly after start.
• Cliff: timestamp must be positive.
• CliffLinear: cliff must be within the inclusive start/end interval.

Return actionable validation errors before building the instruction and reuse the parsed bigint timestamps.

Validation: ran a focused local Node harness against the actual helper/validation/builder source, using Node's TypeScript stripping. All 15 cases pass (7 passed before this change): valid schedules, reversed/equal intervals, non-positive Cliff dates, out-of-range cliffs, inclusive cliff boundaries, and malformed integer input. Also checked bigint output and git diff --check. Full application build/typecheck was not run.

This change was prepared with AI assistance.

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Project	Deployment	Actions	Updated
solana-rewards-program	Ready	Preview	Oct 5, 2026 3:01pm UTC

[dev-jodee]

thanks @Anti83