feat: add Cmd/Ctrl+Enter send setting (#25062)

Adds an Agents General setting to require Cmd/Ctrl+Enter before sending
chat messages. When enabled, plain Enter inserts a newline in agent chat
inputs while the send button remains available.

The preference is now persisted server-side through
`/api/v2/users/{user}/preferences`, alongside the existing user
preference settings, and is applied to both the create-agent input and
existing chat composer. Storybook and API coverage verify the setting,
keyboard behavior, validation, and persistence.

<details>
<summary>Coder Agents notes</summary>

Generated by Coder Agents from a Slack request. Dogfooded with
agent-browser against the Storybook settings and chat input stories.

</details>

coderd/database/dbauthz/dbauthz.go

@@ -4346,6 +4346,17 @@ func (q *querier) GetUserActivityInsights(ctx context.Context, arg database.GetU
 	return q.db.GetUserActivityInsights(ctx, arg)
 }
 
+func (q *querier) GetUserAgentChatSendShortcut(ctx context.Context, userID uuid.UUID) (string, error) {
+	user, err := q.db.GetUserByID(ctx, userID)
+	if err != nil {
+		return "", err
+	}
+	if err := q.authorizeContext(ctx, policy.ActionReadPersonal, user); err != nil {
+		return "", err
+	}
+	return q.db.GetUserAgentChatSendShortcut(ctx, userID)
+}
+
 func (q *querier) GetUserByEmailOrUsername(ctx context.Context, arg database.GetUserByEmailOrUsernameParams) (database.User, error) {
 	return fetch(q.log, q.auth, q.db.GetUserByEmailOrUsername)(ctx, arg)
 }
@@ -7010,6 +7021,17 @@ func (q *querier) UpdateUsageEventsPostPublish(ctx context.Context, arg database
 	return q.db.UpdateUsageEventsPostPublish(ctx, arg)
 }
 
+func (q *querier) UpdateUserAgentChatSendShortcut(ctx context.Context, arg database.UpdateUserAgentChatSendShortcutParams) (string, error) {
+	user, err := q.db.GetUserByID(ctx, arg.UserID)
+	if err != nil {
+		return "", err
+	}
+	if err := q.authorizeContext(ctx, policy.ActionUpdatePersonal, user); err != nil {
+		return "", err
+	}
+	return q.db.UpdateUserAgentChatSendShortcut(ctx, arg)
+}
+
 func (q *querier) UpdateUserChatCompactionThreshold(ctx context.Context, arg database.UpdateUserChatCompactionThresholdParams) (database.UserConfig, error) {
 	u, err := q.db.GetUserByID(ctx, arg.UserID)
 	if err != nil {

coderd/database/dbauthz/dbauthz_test.go

@@ -2842,6 +2842,19 @@ func (s *MethodTestSuite) TestUser() {
 		dbm.EXPECT().UpdateUserCodeDiffDisplayMode(gomock.Any(), arg).Return("always_collapsed", nil).AnyTimes()
 		check.Args(arg).Asserts(u, policy.ActionUpdatePersonal).Returns("always_collapsed")
 	}))
+	s.Run("GetUserAgentChatSendShortcut", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
+		u := testutil.Fake(s.T(), faker, database.User{})
+		dbm.EXPECT().GetUserByID(gomock.Any(), u.ID).Return(u, nil).AnyTimes()
+		dbm.EXPECT().GetUserAgentChatSendShortcut(gomock.Any(), u.ID).Return("modifier_enter", nil).AnyTimes()
+		check.Args(u.ID).Asserts(u, policy.ActionReadPersonal).Returns("modifier_enter")
+	}))
+	s.Run("UpdateUserAgentChatSendShortcut", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
+		u := testutil.Fake(s.T(), faker, database.User{})
+		arg := database.UpdateUserAgentChatSendShortcutParams{UserID: u.ID, AgentChatSendShortcut: "modifier_enter"}
+		dbm.EXPECT().GetUserByID(gomock.Any(), u.ID).Return(u, nil).AnyTimes()
+		dbm.EXPECT().UpdateUserAgentChatSendShortcut(gomock.Any(), arg).Return("modifier_enter", nil).AnyTimes()
+		check.Args(arg).Asserts(u, policy.ActionUpdatePersonal).Returns("modifier_enter")
+	}))
 	s.Run("ListUserChatCompactionThresholds", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
 		u := testutil.Fake(s.T(), faker, database.User{})
 		uc := database.UserConfig{UserID: u.ID, Key: codersdk.ChatCompactionThresholdKeyPrefix + "00000000-0000-0000-0000-000000000001", Value: "75"}