feat(desktop): add new sidebar for desktop

[akarabach]

Issue for this PR

Closes #

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds an alternative sidebar layout ("List" mode) that replaces the two-column
icon strip + panel design with a flat, collapsible project list. Users can switch
between "Classic" and "List" in Settings > General > Sidebar style.

List mode sidebar features:

• Projects shown as collapsible groups with chevron toggle (animated rotation)
• Sessions listed under each project with status indicators, diff stats (+N/-N),
  and relative timestamps (timestamps hidden when diff stats are present)
• Per-project "new session" button revealed on hover
• Per-project "open/add project" button in the header
• Two-step archive: hover reveals archive icon → click changes to "Cancel" →
  click Cancel confirms the archive → mouse-leave resets without archiving
• Sidebar is fully collapsible/toggleable via Cmd+B (no forced-open behavior)
• Resize handle with collapse threshold

Shared components extracted:

• SessionStatusIndicator — self-contained component that computes session
  status (busy/spinner, permission request, error, unseen messages) and renders
  the appropriate indicator. Used by both classic and list sidebars, removing
  duplicated status logic from sidebar-items.tsx.

Settings integration:

• New sidebarStyle setting ("classic" | "list") persisted in general settings
• Dropdown in Settings > General to switch between layouts

How did you verify your code works?

• Tested both sidebar modes in the Tauri desktop app
• Verified toggle (Cmd+B) works in both modes
• Verified archive two-step flow, collapsible groups, status indicators,
  diff stats display, and new session/project actions
• Production build passes (vite build) with no type errors

Screenshots / recordings

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Hey! Your PR title Add New Sidebar List Mode doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍