[Bug] SkillsSection toggle / install buttons stay spinning when IPC throws

## Problem

`SkillsSection.handleToggleEnabled` and `ClawHubTab.handleInstall` both use the pattern:
`setXxx((prev) => new Set(prev).add(key))` → `await window.clawwork.xxx(...)` → `setXxx((prev) => remove key)`.

The cleanup `setXxx` sits after the `await`, outside any `try/finally`. If the IPC call throws, the skill / slug is never removed from the "toggling" / "installing" set and its Power button / Install button stays spinning forever. The user has no way to retry without refreshing the entire section.

## Location

**File:** `packages/desktop/src/renderer/layouts/Settings/sections/SkillsSection.tsx:710-732` (toggle) and `566-583` (install)

```typescript
// handleToggleEnabled
setTogglingKeys((prev) => new Set(prev).add(skill.skillKey));
const newEnabled = skill.disabled;
const res = await window.clawwork.updateSkill(...); // throw → stuck
if (res.ok) { ... } else { ... }
setTogglingKeys((prev) => { const next = new Set(prev); next.delete(skill.skillKey); return next; });

// handleInstall
setInstallingSlugs((prev) => new Set(prev).add(slug));
const res = await window.clawwork.installSkill(gatewayId, { source: 'clawhub', slug }); // throw → stuck
// ... branches ...
setInstallingSlugs((prev) => { const next = new Set(prev); next.delete(slug); return next; });
```

## Fix Approach

Wrap the IPC call in `try/finally` so the set is always cleaned up:

```typescript
setTogglingKeys((prev) => new Set(prev).add(skill.skillKey));
try {
const res = await window.clawwork.updateSkill(...);
if (res.ok) { ... } else { toast.error(...); }
} catch (err) {
console.error('[SkillsSection] toggle failed:', err);
toast.error(t('settings.skillUpdateFailed'));
} finally {
setTogglingKeys((prev) => {
const next = new Set(prev);
next.delete(skill.skillKey);
return next;
});
}
```

Apply the same pattern to `handleInstall`.

## Verification

1. Run `pnpm check` — must pass.
2. Manual: simulate IPC throw during a skill toggle — the button must return to its steady state and a toast should surface the error.

## Context

- **WG:** UI & Design System
- **Priority:** Low (good first issue)
- **Estimated effort:** 15-20 minutes

Open packages/desktop/src/renderer/layouts/Settings/sections/SkillsSection.tsx and inspect handleToggleEnabled around lines 710-732 and handleInstall around lines 566-583. Run pnpm check, then simulate an IPC failure during a toggle or install; done means the button leaves its spinning state and an error toast is shown.

Written by the indexing model from the issue text.