fix(workspace): remember left panel width across collapse

[HandSonic]

Related issue

N/A — regression found by reviewing 2771426 + its persistence helper; described below.

Summary

togglePanelLeft reset the live width to the 260px default on expand, and getPersistableWorkspaceLayout stored 0 while collapsed — a user's custom sidebar width was permanently lost after collapsing (or reloading while collapsed). The layout now tracks lastPanelLeftWidth alongside the live width via a pure nextPanelLeftLayout helper: collapsing remembers the current width, expanding restores it, setPanelLeftWidth keeps it fresh, and persistence/hydration carry the remembered value (legacy layouts without it fall back to previous behavior).

Affected surfaces

• Frontend / Web
• Backend / API / Storage
• Database plugin / Driver
• JCEF / Desktop packaging
• CI / Build / Release
• Documentation only

Verification

• New panelLeftLayout.test.ts (collapse remembers 420, expand restores 420 not 260, default fallback when nothing remembered) and extended workspaceTabPersistence.test.ts (remembered width persisted while collapsed; legacy hydration) — all green.
• Fork CI green: fix(workspace): remember left panel width across collapse HandSonic/Chat2DB#47.

Risk and compatibility

• Public API or stored data: persisted workspace layout gains one additive key; missing key hydrates to legacy behavior.
• Database or driver compatibility: N/A.
• Network, privacy, or security: N/A.
• Community / Local / Pro boundary: N/A.
• Backward compatibility: legacy persisted 0 still means collapsed.

Reviewer map

• Start here: slices/config/initialState.ts (nextPanelLeftLayout) then action.ts and workspaceTabPersistence.ts.
• Failure condition: none expected; default 260 only when nothing was ever remembered.
• Rollback or disable path: revert single commit.

Contributor declaration

• I linked the Issue that defines this change.
• I tested the affected behavior and reported the actual results above.
• I did not include credentials, private data, or generated build output.
• I disclosed substantial AI assistance below, or this PR contains no substantial AI-generated code.

AI assistance: substantial — fix and tests drafted with AI assistance, verified locally.