Windows Terminal Ctrl+V image paste is ignored as empty bracketed paste

## What happened?

On Windows Terminal under WSL, Ctrl+V is owned by the terminal's paste action, so Senpi does not receive the raw `app.clipboard.pasteImage` key. Windows Terminal 1.24 represents an image-only paste as an empty bracketed-paste sequence (`ESC[200~ ESC[201~`).

Senpi's base `Editor` consumes that empty sequence without content, while `CustomEditor` only calls `onPasteImage` for the raw keybinding. The existing WSL PowerShell fallback in `readClipboardImage()` can read the screenshot correctly; it is simply never triggered.

A core reproduction without extensions is to give `CustomEditor.handleInput()` `\x1b[200~\x1b[201~`: `onPasteImage` is called zero times.

This matters because Ctrl+V image paste works in other terminal agents without requiring users to replace their normal terminal paste shortcut.

## Steps to reproduce

1. Run Senpi under WSL in Windows Terminal 1.24 with Ctrl+V bound to terminal paste.
2. Copy a screenshot to the Windows clipboard.
3. Press Ctrl+V in Senpi.

Observed: no image is attached.

## Expected behavior

An image-only empty bracketed paste should call Senpi's existing image paste handler. Normal non-empty text paste must remain unchanged.

A local one-branch patch in `CustomEditor.handleInput()` was verified through a live Senpi TUI: the event attached a readable PNG, while text paste, Ctrl+J, and Enter regression checks remained green.

## Version

Senpi 2026.7.31-2; Windows Terminal 1.24.11911.0; WSL2.

Start in CustomEditor.handleInput() and reproduce the issue with the empty bracketed-paste sequence \x1b[200~\x1b[201~; readClipboardImage() to understand the existing WSL fallback. Done means an image-only empty paste calls onPasteImage while non-empty text paste, Ctrl+J, and Enter remain unchanged.

Written by the indexing model from the issue text.