[Feature Request] Add text selection support in chat input box with shift+arrow and delete operations

Bug Description
Feature request: text selection in the chat input box

Currently shift+arrow selection (selection:extendLeft/Right/Up/Down) exists only
in the Scroll/Transcript contexts, and its only consumer is selection:copy. The
Chat input has no selection state at all — it tracks a text buffer and cursor
position with no anchor — so shift+arrow does nothing there and Backspace/Delete
can only remove one character at a time.

Requested behavior in the Chat context:

• shift+arrow / shift+home / shift+end extends a selection from an anchor
• Backspace / Delete removes the selected range
• typing replaces the selected range
• ctrl+shift+c copies the selection, escape clears it
• expose it as bindable actions (e.g. chat:extendSelectionLeft, chat:deleteSelection)

Why it matters: editing a long multi-line prompt is painful today. The only
options are ctrl+w / ctrl+u / ctrl+k (blunt, cursor-relative) or ctrl+g to bounce
out to an external editor. Terminal mouse drag doesn't help either — that's the
terminal emulator's own selection layer, invisible to the app.

Environment: Claude Code 2.1.235, Windows 11, Windows Terminal / PowerShell

Environment Info

• Platform: win32
• Terminal: windows-terminal
• Version: 2.1.235
• Feedback ID: e4547203-1149-4ca1-b57c-6ad1e9350b04

Errors

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