feat: replace per-cell AI button with global floating panel

The per-cell toolbar AI button (OpenCodeCellActions) is removed. A
single round button is mounted on document.body at the bottom-right of
the JupyterLab shell; clicking it opens a floating panel containing
the same OpenCodeInlinePrompt widget. Clicking again closes it.

The panel resolves the active notebook from app.shell.currentWidget
at open time and re-resolves it whenever currentChanged fires while
the panel is open. The active cell (used by the 📋 插入单元格内容,
↪ Insert, and ⟳ Replace cell actions) is resolved via a new
getActiveCell callback in OpenCodeInlinePrompt, decoupling the prompt
itself from any specific CodeCell instance.

Files:
- src/components/opencode_floating_panel.ts (new) — global panel
- src/components/opencode_cell_actions.ts (deleted) — logic moved up
- src/components/opencode_inline_prompt.ts — getActiveCell option
- src/index.ts — wire floating panel, drop toolbar factory
- style/base.css — .opencode-floating-* styles
- src/__tests__/opencode_floating_panel.spec.ts — new tests (renamed)

All existing functionality preserved: multi-session UI, SSE
streaming, permission/question interaction, history reload.

style/base.css

@@ -4,30 +4,72 @@
     https://jupyterlab.readthedocs.io/en/stable/developer/css.html
 */
 
-/* Single AI action button inside the native Cell toolbar (active cell, top-right). */
-.opencode-cell-actions {
+/* Floating AI launcher (replaces the per-cell toolbar button).
+   Mounted on document.body with position: fixed so it sits above the
+   JupyterLab layout regardless of which dock is active. The panel
+   (when open) renders above the button via column flex. */
+.opencode-floating-root {
+  position: fixed;
+  bottom: 24px;
+  right: 24px;
+  z-index: 1000;  /* above JupyterLab's main UI (z-index tops out around 100) */
   display: flex;
-  align-items: center;
+  flex-direction: column;
+  align-items: flex-end;
+  gap: 8px;
 }
 
-.opencode-cell-actions .opencode-btn {
+.opencode-floating-button {
+  width: 48px;
+  height: 48px;
+  border-radius: 50%;
   border: none;
-  background: transparent;
-  padding: 0 6px;
-  font-size: var(--jp-ui-font-size1);
-  line-height: 20px;
+  background: var(--jp-brand-color1, #1976d2);
+  color: white;
+  font-size: 20px;
   cursor: pointer;
-  white-space: nowrap;
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.2);
+  transition: transform 0.15s ease, box-shadow 0.15s ease;
+  padding: 0;
+  line-height: 1;
+}
+.opencode-floating-button:hover {
+  transform: scale(1.05);
+  box-shadow: 0 6px 16px rgba(0, 0, 0, 0.25);
+}
+.opencode-floating-button:focus {
+  outline: 2px solid var(--jp-brand-color2, #42a5f5);
+  outline-offset: 2px;
 }
 
-.opencode-cell-actions .opencode-btn:hover:enabled {
-  background: var(--jp-layout-color2);
-  border-radius: 2px;
+.opencode-floating-panel {
+  width: 480px;
+  max-width: calc(100vw - 48px);
+  max-height: 60vh;
+  overflow: hidden;
+  display: flex;
+  flex-direction: column;
+  background: var(--jp-layout-color1, #fff);
+  border: 1px solid var(--jp-border-color2, #ddd);
+  border-radius: 6px;
+  box-shadow: 0 8px 24px rgba(0, 0, 0, 0.18);
+}
+.opencode-floating-panel[hidden] {
+  display: none;
 }
 
-.opencode-cell-actions .opencode-btn:disabled {
-  opacity: 0.4;
-  cursor: default;
+/* The inline prompt inside the floating panel should fill it: drop
+   the standalone panel's own margin/border, and let the panel's
+   scrollable region take over. */
+.opencode-floating-panel .opencode-inline-prompt {
+  margin: 0;
+  border: none;
+  border-radius: 0;
+  flex: 1;
+  overflow-y: auto;
+}
+.opencode-floating-panel .opencode-inline-history {
+  max-height: 40vh;
 }
 
 /* Inline prompt panel attached to the cell when the AI button is clicked. */