feat(sidebar): open projects in a new browser tab

[wowayou]

Summary

Add Open in new tab (T) to a project group's ⋯ menu, immediately after Open in Files. It opens a fresh composer for that project without switching the original browser tab or selecting an arbitrary previous conversation.

• Current project: use the sidebar's selected directory/worktree.
• Another project, including a pinned project without sessions: use its root.
• The original tab retains its selected session, directory, draft, file preview and group expansion.
• The action creates, copies, prompts and stops no session. Project-header expand/collapse and same-tab + keep their existing behavior.

Closes #1207

Reuse and scope

Only three production files change:

• components/SessionSidebar.tsx reuses SidebarMenu, the existing sidebar.openInNewTab translations, OpenInNewTabIcon, shortcut handling and the session menu's synchronous window.open(..., "_blank", "noopener,noreferrer") pattern.
• lib/new-session-context.ts extracts the existing group's + target rule into newSessionTargetForProject(), shared by + and the new menu action.
• lib/session-links.ts adds the cwd counterpart to its session link: newSessionDeepLink(). The existing ?cwd= validation, fresh-composer initialization and per-tab restore behavior are unchanged. The relative link keeps origin/path and removes stale query parameters and fragments.

No new navigation framework, app-internal tabs, session state store, backend API, dependency, or setting. Browser Back/Forward, changes to New, project filtering and the broader conversation-finding discussion (#1201) are outside this PR. The separate static demo is unchanged.

Tests

• node_modules/.bin/tsc --noEmit: PASS.
• npm run lint: PASS.
• npm test: PASS — 2630 tests, 2629 passed, 1 skipped, 0 failures on the full rerun.
• PI_OFFLINE=1 PI_WEB_SKIP_VERSION_CHECK=1 npm run test:e2e: PASS — 36 checks on the full rerun (27 existing + 9 new).
• A focused run of the new project-navigation browser checks also passed.
• git diff --check: PASS.

The new browser checks are integrated into the existing e2e/run.mjs server lifecycle and temporary agent directory, and reuse the existing HTML file-panel fixture and sidebar helpers. They cover desktop and mobile Chromium, a real linked worktree with spaces/non-ASCII/&/# in its path, another project, an empty pinned project, click and keyboard activation, reload vs remembered sessions, source draft and file-preview preservation, the unchanged + action, and an unavailable directory. Requests and session IDs are checked to rule out session creation/copying/prompting/stopping. Unit tests also cover Windows/UNC path encoding and sub-path URL resolution.

Initial-run failures, retained for transparency

• The first unit-suite run, alongside lint and type checking, failed in the unchanged lib/project-command-env.test.mjs timeout test because its descendant's survivor.pid had not been written. Running the entire unmodified suite again on its own passed.
• The first full browser run stopped with Chromium net::ERR_NETWORK_CHANGED during an existing session-load step, before the new checks. The entire unmodified browser suite rerun passed.

No assertions were relaxed or retry logic added to address either failure. All browser runs used temporary fixture sessions; no real sessions or inference requests were used. The existing installed Pi Web service was left running untouched.

Documentation

Updated docs/agents/sessions.md, the English/Chinese worktree guides, and e2e/README.md to describe the new entry point, its directory rule and regression coverage.

Verification limits

Verified on Linux with headless Chromium, including a mobile viewport/touch emulation. Real mobile devices, macOS, Windows, Firefox and Safari were not exercised; Windows path handling is unit-tested only. No development build was run.