[L2-06] Add isolated local workspaces

[dkritarth]

Outcome

Support multiple local research libraries with explicit switching and no data leakage.

Dependencies

Blocked by #39.

Scope

Choose and document workspace model before migration. Create/list/rename/switch workspace; isolate DB and paper roots; handle active chats/sessions during switch. No cloud synchronization.

Acceptance criteria

• Workspace identity and data-root rules are explicit and path-safe.
• Create/switch/rename works and survives restart.
• Papers, collections, notes, highlights, chats, and trash remain isolated.
• Switching disposes or safely rebinds ACP sessions and UI tabs.
• Existing single-workspace data migrates without loss.
• Invalid/missing workspace path degrades safely.
• Two-workspace live isolation test passes.

Live verification

Create two workspaces, ingest distinct papers, create same-named collections/notes, switch repeatedly, restart, verify isolation, test switch during open chat, inspect files and console.

Required evidence

Focused PR uses Closes #40. Provide failing test/baseline, focused and full tests, typecheck, build, diff check, exact live Electron steps, screenshots where applicable, console state, restart/persistence result, and limitations. Reviewer independently exercises live behavior.