feat(editor): add snippet paste dialog (alt+v)

[EduardF1]

Summary

• Adds a new Snippet Paste dialog triggered by Alt+V from the chat editor.
• A full-screen overlay textarea appears where users can paste or type multi-line code.
• Pressing Ctrl+D (done) wraps the content in a fenced code block and inserts it at the cursor in the editor; Esc cancels without inserting.
• Adds Editor.PasteSnippet to the FullHelp key bindings so users can discover it via Ctrl+G.

Motivation

The chat editor currently handles pasted multi-line text in two ways: short pastes go directly into the textarea, and long pastes (>10 lines) become anonymous file attachments. Neither flow explicitly wraps content in code fences, which is the most common intent when a user pastes a snippet they want the model to analyse or edit.

This mirrors the Alt+V snippet-paste workflow familiar from Claude Code.

Test plan

• Run go test ./internal/ui/dialog/... -run TestSnippet — 5 new unit tests all pass.
• Run go test ./... — all existing tests continue to pass (pre-existing TestDispatch_BashShebang fails on Windows due to missing WSL /bin/bash; unrelated to this change).
• Manual: open Crush, press Alt+V → overlay appears; type code → Ctrl+D → fenced code block inserted into the editor.
• Manual: press Esc in overlay → dialog closes, nothing inserted.
• Manual: Ctrl+G in editor → "alt+v paste snippet" appears in FullHelp.

Files changed

File	Change
internal/ui/dialog/snippet.go	New Snippet dialog (textarea overlay)
internal/ui/dialog/snippet_test.go	5 unit tests
internal/ui/dialog/actions.go	New ActionInsertSnippet action
internal/ui/model/keys.go	New Editor.PasteSnippet binding (alt+v)
internal/ui/model/ui.go	Key handler + action handler + FullHelp entry

🤖 Generated with Claude Code

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