feat(workspace): improve session panel resizing

[openai0229]

Related issue

Closes #2260

Summary

Replaces the custom Open Sessions resize handlers with the existing react-split-pane component already used elsewhere in Chat2DB. The sessions panel keeps a 220 px default height and remains bounded between 72 px and 420 px, while the standard divider provides a discoverable resize cursor and interaction.

Affected surfaces

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

Verification

• Commands and results: yarn eslint src/pages/main/workspace/components/WorkspaceExplorer/index.tsx src/pages/main/workspace/components/WorkspaceExplorer/style.ts passed; git diff origin/main...HEAD --check passed.
• Manual verification: Verified in the Community desktop client against the local Web runtime. The requester tested the divider and confirmed the interaction works as expected.
• UI evidence: Manual JCEF verification completed; no separate image artifact attached.

Risk and compatibility

• Public API or stored data: N/A; no API or storage changes.
• Database or driver compatibility: N/A; no database or driver changes.
• Network, privacy, or security: N/A; no network or trust-boundary changes.
• Community / Local / Pro boundary: Community frontend only.
• Backward compatibility: Preserves the existing default panel height and resize bounds while replacing the interaction implementation.

Reviewer map

• Start here: WorkspaceExplorer/index.tsx, specifically the horizontal SplitPane around the local file tree and Open Sessions panel.
• Failure condition: The divider does not show the row-resize cursor, cannot resize smoothly, or allows the sessions panel outside the 72-420 px bounds.
• Rollback or disable path: Revert this commit to restore the previous custom pointer handlers and styles.

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: OpenAI Codex assisted with implementation review, focused verification, and PR preparation.