feat(web): persist collapsed workspace groups to localStorage

[wbxl2000]

Related Issue

No related issue — this is a small UX polish for the web sidebar.

Problem

When the web page is reloaded, every workspace group in the sidebar expands back to its default state. Users who keep several workspaces collapsed have to re-fold them on each visit.

What changed

The collapsed set of workspace ids is now persisted to localStorage (key kimi-web.collapsed-workspaces) and restored on mount. The fold/unfold toggle writes the updated set back, so the sidebar looks the same after a refresh. This is UI-only state with no server-side source of truth, so localStorage is the right place — consistent with how the sidebar already persists its own width/collapsed state, hidden workspaces, and unread dots.

• src/lib/storage.ts: new loadCollapsedWorkspaces / saveCollapsedWorkspaces helpers with array/shape validation.
• src/components/Sidebar.vue: initialize the collapsed set from storage and save on every toggle.
• test/storage-logic.test.ts: covers missing key, round-trip, iterable input, and malformed/non-string values.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update. (No user-facing docs to update for this behavior.)

[pkg-pr-new]

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npx https://pkg.pr.new/@moonshot-ai/kimi-code@5d54ebd

commit: 5d54ebd