feat: move notes tools into studio-style content cards

[mrsibe]

What does this PR do?

将最右侧笔记面板的四个创建入口从顶栏移入内容区，采用 NotebookLM Studio 风格的紧凑 2×2 工具卡片：创建笔记、思维导图、生成题目、Anki 卡片。

Why?

让入口具有可见文字，减少顶栏拥挤，并让工具和已有产物在同一内容区连续呈现。

What changed?

• 顶栏只保留“笔记”标题；工具卡片和现有条目列表共享滚动区域。
• 新增限定用于这四张卡片的淡蓝、淡紫、淡绿、淡沙色语义 token，包含明暗主题定义；同步扩展 DESIGN.md，不复用图表色、不放宽其他区域的配色规则。
• 保持中性图标/文字、14px 标签、8px 圆角、细边框、无阴影；悬停叠加状态填充并保留彩色底色。
• 保留所有现有操作、生成状态、对话框、列表排序与编辑器/未保存提醒。“创建笔记”仍为新建空白笔记，不新增 AI 生成功能。
• 同步中英文空态提示，添加工具映射、token 范围、悬停样式及文案回归测试。

How was this tested?

• npm run check:design — passed.
• npm run typecheck — passed.
• npm test — 496 tests passed.
• npm run build — passed.
• Changed-file ESLint — 0 errors; one existing react-hooks/exhaustive-deps warning in unchanged editor save logic.
• Changed-file Prettier check and git diff --check — passed.
• Independent subagent review identified and corrected hover fill replacement and a documentation/token mismatch.
• Temporary Electron harness rendered the actual NotePanel with mocked IPC/store at 260px panel width in a short 300px window, across light/dark, English/Chinese, empty/populated states. Verified four cards, no horizontal overflow, label wrapping and shared scrolling. Verified hover retains opaque fill, visible keyboard Tab focus ring, and click dispatch for mocked create-note/mind-map callbacks.
• This is isolated UI verification, not a full desktop generation/editor workflow test. Native Enter activation was not verified by the temporary harness. The environment lacks Chinese glyph fonts, so screenshots use English. No packaged-app smoke test is needed (renderer-only change).

Screenshots

Actual component with mocked local items; intentionally narrow/short to show wrapping. Temporary harness/scripts are not included in this PR.

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Checklist

• I have reviewed my own changes.
• npm run typecheck passes.
• npm run build passes.
• Relevant isolated UI behavior and regression tests pass.
• I have not included unrelated changes.
• I have updated documentation when necessary.
• Desktop/build/packaging changes: not applicable.