fix(desktop): keep session tab strips visible

[Dolverin]

What does this PR do?

Keeps the Desktop session-tab strip visible for every chat switcher zone (workspace and session-tile:*). Today a lone workspace auto-hides its strip, and a persisted headerHidden: true can make that state survive restarts. Because the only Show header action lives in the hidden strip, the user is left with no in-app recovery path.

This rebased successor to #89225 keeps that navigation surface available while addressing its review feedback:

• reuses the existing isSessionStripPane classifier instead of introducing a second pane-id authority;
• preserves paneChrome.headerVeto for full-page views;
• removes Hide header only from session switcher zones, while tool/side zones retain it;
• compensates the first tap's minimize before refusing a session-strip double-tap hide, so minimizable session tiles do not collapse.

The change is complementary to #91514 / #86278, which prevent tab double-clicks from forwarding the strip-hide gesture. This PR owns the remaining lone-workspace, persisted-state, menu, and strip-background paths.

Co-authored with and based on the policy work in #89225 by @leonphull; this branch updates the implementation to current main and incorporates the blocking review feedback.

Related Issue

Fixes #89350
Related: #79518, #89225, #91514

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• renderer/lone-header.ts: centralize header visibility precedence for session vs non-session zones.
• renderer/tree-group.tsx: keep session strips visible, omit their dead-end hide action, and refuse persisted/double-tap hiding while preserving minimize compensation.
• Renderer tests: cover lone workspace, persisted hide, full-page veto, non-session policy, context-menu behavior, and the synthesized double-tap path.

How to Test

1. Start Desktop with a layout containing only workspace in grp-main and no explicit headerHidden; confirm the workspace tab and + remain visible.
2. Start with grp-main.headerHidden: true; confirm the session strip still renders and Hide header is absent from its context menu.
3. Double-tap the strip background of a minimizable session-tile:* zone; confirm the zone is not left minimized and headerHidden: true is not persisted.
4. Open a full-page view; confirm headerVeto still suppresses the strip.
5. Open a lone tool zone such as terminal; confirm its existing Hide header action still works.

Automated verification:

• vitest run --project ui src/components/pane-shell/tree: 24 files, 125 tests passed.
• Sabotage run with the session-strip resolver disabled: 3 new regressions fail; restoring the fix returns the suite to green.
• npm run typecheck: clean.
• npm run lint: 0 errors (repository baseline warnings only).
• npm run build: production renderer + Electron bundles built successfully.
• git diff --check: clean.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run the relevant Desktop test suite; Python tests are unaffected by this renderer-only change
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: Windows 11 packaged Desktop plus jsdom/Vitest regression suite

Documentation & Housekeeping

• Documentation update N/A — renderer behavior and inline policy comments updated
• cli-config.yaml.example N/A — no config change
• CONTRIBUTING.md / AGENTS.md N/A — no architecture or workflow change
• Cross-platform impact considered — pure renderer policy, no OS-specific API
• Tool descriptions/schemas N/A — no tool change

Screenshots / Logs

Live packaged Windows reproduction showed grp-main containing only workspace with no headerHidden; the left strip was absent while another session group still displayed tabs. Setting grp-main.headerHidden = false through DevTools restored the workspace tab and New session tab affordance immediately. The regression suite now enforces that state without requiring DevTools recovery.

[OutThisLife]

Right diagnosis — a session tile alone in a zone has nothing but the strip carrying its ✕.

#91714 makes that a named invariant: resolveTabStripVisible treats a closeable placement: 'main' pane as stranded without a strip, and stranding outranks an explicit never, so no setting can produce a zone that answers no gesture. It's part of a rebuild of headerHidden covering nine open PRs in this area. Closing in favor of that.