fix(ui): 左栏图标轨展开/折叠同元素沉底 + 折叠箭头移右边线垂直居中

- SessionRail: 图标轨恒显(展开态靠左 14px 边距,折叠态居中),margin-top:auto 沉底
  ——展开/折叠切换时图标 Y 位置一致
- SessionPanel: 移除 S4 文字「设置」按钮(图标轨已覆盖展开态入口)
- base.css: side-handle-btn 从左下角移到左栏右边线垂直居中(展开 left:260px,
  折叠 left:64px,过渡动画),图标方向由 AppShell 状态切换(收起→右/展开→左)
- inner flex:1 明确高度链,图标轨在内容滚动时始终可见

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

ui/src/components/session/SessionPanel.vue

@@ -139,14 +139,6 @@ const doDelete = async () => {
         </template>
       </div>
 
-      <!-- 展开态设置入口（S4）：折叠态齿轮在 SessionRail，展开态在此 -->
-      <button class="panel-settings-btn" type="button" :title="T.settingsTitle" @click="$emit('goto-settings')">
-        <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
-          <circle cx="12" cy="12" r="3" />
-          <path d="M19.4 15a1.65 1.65 0 0 0 .33 1.82l.06.06a2 2 0 0 1 0 2.83 2 2 0 0 1-2.83 0l-.06-.06a1.65 1.65 0 0 0-1.82-.33 1.65 1.65 0 0 0-1 1.51V21a2 2 0 0 1-2 2 2 2 0 0 1-2-2v-.09A1.65 1.65 0 0 0 9 19.4a1.65 1.65 0 0 0-1.82.33l-.06.06a2 2 0 0 1-2.83 0 2 2 0 0 1 0-2.83l.06-.06a1.65 1.65 0 0 0 .33-1.82 1.65 1.65 0 0 0-1.51-1H3a2 2 0 0 1-2-2 2 2 0 0 1 2-2h.09A1.65 1.65 0 0 0 4.6 9a1.65 1.65 0 0 0-.33-1.82l-.06-.06a2 2 0 0 1 0-2.83 2 2 0 0 1 2.83 0l.06.06a1.65 1.65 0 0 0 1.82.33H9a1.65 1.65 0 0 0 1-1.51V3a2 2 0 0 1 2-2 2 2 0 0 1 2 2v.09a1.65 1.65 0 0 0 1 1.51 1.65 1.65 0 0 0 1.82-.33l.06-.06a2 2 0 0 1 2.83 0 2 2 0 0 1 0 2.83l-.06.06a1.65 1.65 0 0 0-.33 1.82V9a1.65 1.65 0 0 0 1.51 1H21a2 2 0 0 1 2 2 2 2 0 0 1-2 2h-.09a1.65 1.65 0 0 0-1.51 1z" />
-        </svg>
-        {{ T.settingsTitle }}
-      </button>
     </div>
 
     <!-- 折叠后的图标轨（FR-SHELL-004 的 B2 侧内容） -->
@@ -196,22 +188,6 @@ const doDelete = async () => {
   flex-direction: column;
   gap: 10px;
 }
-.panel-settings-btn {
-  display: flex;
-  align-items: center;
-  gap: 8px;
-  width: calc(100% - 32px);
-  margin: 0 16px 12px;
-  padding: 8px 12px;
-  border: 1px solid var(--pa-border, #e5e7eb);
-  border-radius: var(--pa-r-pill, 8px);
-  background: transparent;
-  color: var(--pa-text, #111827);
-  font-size: 13px;
-  cursor: pointer;
-}
-.panel-settings-btn svg { width: 16px; height: 16px; flex: none; }
-.panel-settings-btn:hover { background: var(--pa-surface-2, #f3f4f6); }
 
 .new-task-btn {
   display: flex;

ui/src/components/session/SessionRail.vue

@@ -37,9 +37,11 @@ const emit = defineEmits(['open', 'goto-kg', 'goto-settings']);
 .collapsed-rail {
   display: flex;
   flex-direction: column;
-  align-items: center;
+  align-items: flex-start;  /* 展开态：图标靠左（原样图标，14px 对齐内容边距） */
   gap: 14px;
-  padding-top: 14px;
+  width: 100%;
+  padding: 0 0 14px 14px;
+  margin-top: auto;          /* 沉底：与折叠态同一元素，切换时 Y 高度不变 */
 }
 .rail-item {
   display: inline-flex;

ui/src/styles/base.css

@@ -171,12 +171,12 @@ html.drawer-open { overflow: hidden; }
   min-width: 0; overflow: hidden;
   transition: all .3s var(--pa-ease-out);
 }
-.session-panel .inner { display: flex; flex-direction: column; min-height: 0; width: 260px; transition: opacity .2s var(--pa-ease); }
+.session-panel .inner { display: flex; flex-direction: column; min-height: 0; width: 260px; flex: 1; transition: opacity .2s var(--pa-ease); }
 .side-collapsed .session-panel .inner { opacity: 0; pointer-events: none; }
-.session-panel .collapsed-rail {
-  display: none; flex-direction: column; align-items: center; gap: 14px; padding-top: 14px;
+.side-collapsed .session-panel .collapsed-rail {
+  align-items: center;   /* 折叠态：64px 轨内居中，图标原样 */
+  padding: 0 0 14px;     /* 折叠态：去掉展开态的左 14px（底 14px 两态一致） */
 }
-.side-collapsed .session-panel .collapsed-rail { display: flex; }
 .rail-item {
   display: inline-flex; align-items: center; justify-content: center;
   width: 34px; height: 34px; border-radius: var(--pa-r-md);
@@ -694,16 +694,20 @@ html.drawer-open { overflow: hidden; }
 
 /* ---------- 折叠把手（UI_SPEC §3，FR-SHELL-004） ---------- */
 .side-handle-btn {
-  position: absolute; left: 0; bottom: 40px; z-index: 7;
+  /* 位于左栏右边线、侧栏区域（顶栏与底栏之间）垂直居中；跨线半嵌 */
+  position: absolute; left: 260px; z-index: 7;
+  top: calc(var(--topbar-h) + (100% - var(--topbar-h) - var(--statusbar-h)) / 2);
+  transform: translate(-50%, -50%);
   width: 26px; height: 26px;
   border: 1px solid var(--pa-hairline);
   background: var(--pa-surface-1);
   border-radius: var(--pa-r-md);
   display: inline-flex; align-items: center; justify-content: center;
   color: var(--pa-ink-muted);
+  transition: left .3s var(--pa-ease-out), background .2s var(--pa-ease), color .2s var(--pa-ease);
 }
-.side-handle-btn svg { width: 13px; height: 13px; transition: transform .3s var(--pa-ease-out); }
-.side-handle-btn.collapsed svg { transform: rotate(180deg); }
+.side-handle-btn svg { width: 13px; height: 13px; }
+.app-shell.side-collapsed .side-handle-btn { left: 64px; }
 
 /* ---------- 通用抽屉（UI_SPEC §5 / §7：drawer / drawer-scrim） ---------- */
 .drawer-scrim {