feat: tab-local selection and per-tab terminal isolation

[gbasin]

This PR makes agentboard multi-tab behavior predictable across selection state, terminal switching, and session list animations.

Summary

• Session selection persistence is now tab-local after initial hydration.
• PTY terminal switching now stays isolated per websocket connection, preventing tabs from fighting over tmux window focus.
• Safari session list animation fallback was hardened, including drag displacement behavior.
• Redundant setSessions snapshots are deduped without dropping lastActivity updates.
• Patch version bumped to 0.2.19.

What changed

Client state and persistence

• Added createTabStorage(expectedKey) and wired sessionStore persist key agentboard-session to tab-scoped storage.
• Kept persisted scope narrow to selectedSessionId.
• Added tests for hydration, per-tab read isolation, write-through, remove behavior, and key guardrails.

Terminal isolation

• Updated PtyTerminalProxy to rewrite base-session switch targets (for example agentboard:@2) to grouped per-connection targets (for example agentboard-ws-<connection>:@2).
• Added isolated proxy test coverage for grouped target rewriting.

Session list behavior and Safari

• Added Safari-specific fallback for list transitions and layout handling.
• Ensured DnD transforms are still applied in Safari fallback so non-dragging items displace correctly during sorting.
• Added targeted transform composition tests.

Snapshot update behavior

• Deduped redundant setSessions updates to reduce animation churn.
• Corrected equality logic to preserve lastActivity updates and use order-insensitive session comparisons.
• Added regression test for lastActivity-only updates.

Notes

• Cross-tab live sync of selected session remains intentionally disabled after hydration.
• New tabs/reloads still hydrate from the shared latest persisted default.

Validation

• bun run lint
• bun run typecheck
• bun run test