[FEATURE]: Support full-width `！` to toggle Shell/Prompt mode in web UI and TUI

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Problem

OpenCode uses a leading ! to enter Shell mode (and back to Prompt mode). On CJK input methods (Chinese IME, etc.), typing the ! key usually produces the full-width character ！ instead of the half-width !. As a result:

• In the TUI, entering ！ls is not recognized as a shell command — it's sent to the model as normal text instead of switching into Shell mode.
• The web UI has the same issue: ！ does not trigger the shell-mode toggle.

This is the same class of friction reported for slash commands in #11485 (where 、 from a CJK IME broke /commands), but here specifically for the shell-mode ! toggle.

Requested feature

Treat the full-width ！ (U+FF01) as equivalent to ! for toggling Shell/Prompt mode, in both the web UI and the TUI:

• ！ls should behave exactly like !ls (enter Shell mode, run ls)
• After executing a shell command, ！ (like !) should toggle back to Prompt mode

Optionally, this could be made configurable (e.g., a tui.json / config setting listing alternative prefixes, mirroring the general "alternative command prefix" idea from #11485) so it also covers other CJK punctuation.

Why

Shell mode is a core workflow (running deterministic commands). For users who type with a CJK IME, the full-width key is the default output, making the ! shortcut effectively unusable without switching IMEs. Supporting ！ removes that friction for both the web UI and the TUI.

Related

• #11485 (closed, auto-stale): alternative command prefix for non-English IMEs (、 for slash commands) — same underlying IME issue, different prefix.