feat: Enter in the inline prompt textarea submits; Shift/Ctrl+Enter newline

Chat-input convention for the inline prompt's textarea:
  - Enter (no modifier): preventDefault() + click the send button
    (which validates non-empty text and calls onSubmit with the
    current text + the selected provider/model).
  - Shift+Enter / Ctrl+Enter: do NOT preventDefault, so the textarea
    inserts a newline (the default browser behavior).

Implementation:
  - opencode_inline_prompt.ts: add a keydown listener on the textarea
    in the constructor (right after the send button setup, so
    this._sendBtn is already assigned and the closure can call
    this._sendBtn.click()).

Tests:
  - 'Enter in the textarea submits; Shift+Enter does not':
    dispatches keydown Enter on the textarea and asserts the
    onSubmit jest.fn() is called once with the textarea value and the
    current provider/model. Then dispatches Shift+Enter and
    Ctrl+Enter and asserts onSubmit is NOT called.

Verification: pytest 37/37, jest 36/36 (was 35, +1).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/components/opencode_inline_prompt.ts

@@ -125,6 +125,14 @@ export class OpenCodeInlinePrompt extends Widget {
       const modelId = this._modelSelect?.value || undefined;
       options.onSubmit(text, providerId, modelId);
     });
+    // Chat-input convention: Enter submits, Shift+Enter / Ctrl+Enter
+    // insert a newline (textarea default).
+    this._textarea.addEventListener('keydown', (e) => {
+      if (e.key === 'Enter' && !e.shiftKey && !e.ctrlKey) {
+        e.preventDefault();
+        this._sendBtn.click();
+      }
+    });
     this._cancelBtn = document.createElement('button');
     this._cancelBtn.className = 'opencode-btn-cancel';
     this._cancelBtn.textContent = '取消';