feat: Base64 error toasts, multi-line input, and multi-format timestamp conversion

[Copilot]

Base64 decode errors were silently swallowed with only an inline text hint. Timestamp conversion only emitted Unix seconds with no support for milli/nanosecond inputs or outputs.

Base64

• Swapped inline error state for sonner toast notifications (toast.error(...)) on encode/decode failure
• Replaced single-line <Input> with <Textarea rows={6}> for both input and output — handles multi-line plaintext and wrapped Base64
• Added src/components/ui/textarea.tsx styled to match the existing Input component

Timestamp

• Auto-detects input unit by digit length: ≤10 → seconds, ≤13 → milliseconds, >13 → nanoseconds
• Both conversion directions now emit all four formats via a shared TimestampResultTable component:

Label	Example
ISO 8601	2023-11-14T22:13:20.000Z
Unix (s)	1700000000
Unix Milli (ms)	1700000000000
Unix Nano (ns)	1700000000000000000

• Nanosecond output uses BigInt arithmetic (BigInt(ms) * 1_000_000n) to avoid float precision loss beyond Number.MAX_SAFE_INTEGER

App

• Added <Toaster richColors /> from sonner outside the router, wrapped in a fragment