[Design Proposal] Configurable theme/skin module (Task #1 from #2291)

## Summary

Design proposal for **Task 1** from #2291 — a **configurable theme/skin module** to enhance product branding and appearance customization (P0).

This issue is opened to discuss the design before submitting a PR, per the contributing guide. I have already [commented to claim Task 1](https://github.com/agentscope-ai/QwenPaw/issues/2291#issuecomment-4927764231).

## Current state

- `console/src/contexts/ThemeContext.tsx` already provides **mode switching**: `light` / `dark` / `system`, persisted in `localStorage` (`qwenpaw-theme`), listening to `prefers-color-scheme`, toggling a `dark-mode` class on ``.
- `console/src/components/ThemeToggleButton/index.tsx` exposes a dropdown for those three modes, rendered in `console/src/layouts/Header.tsx`.
- In `console/src/App.tsx`, antd `ConfigProvider` mounts with `algorithm` driven by `isDark`, but `token.colorPrimary` is **hardcoded** to `#FF7F16`:
```tsx
theme={{
algorithm: isDark ? antdTheme.darkAlgorithm : antdTheme.defaultAlgorithm,
token: { colorPrimary: "#FF7F16" },
}}
```

So the brand color is fixed and not user-configurable; there is no concept of a "skin" beyond light/dark.

## Goal

Let users pick a **preset skin** (brand palette) in addition to light/dark/system — without breaking existing behavior and without a heavy free-form color picker.

## Proposed design — preset skin module

1. **Skins are preset objects**, e.g.:
```ts
type Skin = { key: string; colorPrimary: string; colorInfo?: string; ... };
const SKINS: Skin[] = [
{ key: "default", colorPrimary: "#FF7F16" }, // current orange, kept as default
{ key: "blue", colorPrimary: "#1677ff" },
{ key: "purple", colorPrimary: "#722ed1" },
{ key: "green", colorPrimary: "#52c41a" },
];
```
2. **Extend `ThemeContext`** with a `skin` field + `setSkin`, persisted to `localStorage` (`qwenpaw-skin`), defaulting to `default` (orange) so existing visuals are unchanged.
3. **Drive antd tokens from the skin** in `App.tsx` — `colorPrimary` (and optionally `colorInfo`/border tokens derived via antd's built-in palette generation) replace the hardcoded value. `algorithm` still follows `isDark`.
4. **Expose a CSS variable** `--qwenpaw-brand-primary` (and a couple of companions) on `:root`, updated when the skin or resolved theme changes, so non-antd surfaces (`.module.less` files) can opt in gradually without a big-bang rewrite.
5. **UI**: add a compact skin picker (swatches) alongside the existing `ThemeToggleButton` in `Header.tsx`, reusing its dropdown pattern.
6. **i18n + tests**: add `theme.skin.*` strings to all locales; extend `ThemeContext.test.tsx` to cover skin persistence and token/CSS-variable application.

## Out of scope (intentionally)

- Free-form custom color picker / hex input (can be a follow-up).
- Full design-token refactor / migrating every hardcoded color (too large for one PR; the CSS variable is the enabler for a later incremental migration).
- Per-agent skins.

## Alternatives considered

- **A. Free-form custom colors** — more powerful, but bigger UI surface, more validation/i18n/testing, and maintainers may prefer a curated set. Better as a follow-up.
- **B. Token-only refactor (no user-facing feature)** — useful long-term, but doesn't satisfy the "configurable" intent of Task 1 on its own.

## Test plan

- Unit: `ThemeContext` persists/restores `skin`; selecting a skin updates the exposed tokens and `--qwenpaw-brand-primary`.
- Manual: switching skin visibly changes the brand color across the Console; light/dark/system still works independently; selection survives reload.
- No regression: default skin reproduces today's `#FF7F16` look exactly.

Happy to adjust the preset list, the CSS-variable naming, or the picker placement based on feedback.