fix: improve two-stage private key input UX (32+32 → 58+6 split) (#942)

## Problem

Users reported that the 32+32 character split design is not user-friendly:
1. ❌ Second stage still requires entering 32 characters - hard to count
2. ❌ Need to count many characters in both stages
3. ❌ Easy to make mistakes when counting

## Solution

Change the split from 32+32 to **58+6**

**Stage 1**: 58 characters
- Enter the majority of the key (90%)
- Easy to copy/paste the prefix

**Stage 2**: 6 characters
- ✅ Only need to count last 6 chars (very easy)
- ✅ Quick verification of key suffix
- ✅ Reduces user errors

## Changes

```typescript
// Old: Equal split
const expectedPart1Length = Math.ceil(expectedLength / 2)  // 32
const expectedPart2Length = expectedLength - expectedPart1Length  // 32

// New: Most of key + last 6 chars
const expectedPart1Length = expectedLength - 6  // 58
const expectedPart2Length = 6  // Last 6 characters
```

## Test plan

✅ Frontend builds successfully (npm run build)
✅ User-friendly: Only need to count 6 characters
✅ Maintains security: Two-stage input logic unchanged

Co-authored-by: the-dev-z <the-dev-z@users.noreply.github.com>

Author: the-dev-z

web/src/components/TwoStageKeyModal.tsx

@@ -63,8 +63,10 @@ export function TwoStageKeyModal({
   const stage1Ref = useRef<HTMLInputElement>(null)
   const stage2Ref = useRef<HTMLInputElement>(null)
 
-  const expectedPart1Length = Math.ceil(expectedLength / 2)
-  const expectedPart2Length = expectedLength - expectedPart1Length
+  // UX improvement: Use 58 + 6 split (most of the key + last 6 chars)
+  // Advantage: Second stage only requires entering 6 characters, much easier to count
+  const expectedPart1Length = expectedLength - 6  // 64 - 6 = 58
+  const expectedPart2Length = 6  // Last 6 characters
 
   useEffect(() => {
     if (isOpen && stage === 1 && stage1Ref.current) {