feat(app): let new tabs use the last selected project

[opencode-agent]

Summary

• Add a Preferences setting to open new session tabs in the current tab directory (default) or the last selected project.
• Count project selection on Home toward the last selected project; keep existing Home behavior in the default mode and preserve a current-tab fallback when no saved project exists.
• Retain the source model when staying on the same server, but use the destination server defaults when crossing servers.

Checks

• App typecheck, 50 focused unit tests, and 4 Chromium regression tests passed.
• Full monorepo check is blocked by @opencode/posts: the Host Node 18 is below Astro’s supported version; forcing Bun instead terminates its typecheck (137). The separate E2E typecheck reports three unchanged errors in performance probes and session-queue.spec.ts.

Requested by: @Brendonovich (Brendan via Slack)