feat: add session history browser to desktop sidebar

[JohnFox191]

Summary

• Adds a session history browser to the desktop app sidebar, allowing users to browse and switch between past sessions organized by directory or project
• Includes directory tree and project tree views with session count badges, pagination, and session switching
• Fixes a count mismatch where the Show sessions (N) badge in directory view returned fewer sessions than actually existed (project-based fetch was used when directory matched a worktree)

Changes

• New session-browser component (panel, directory-node, session-node)
• New API endpoints for listing directories and sessions by project/directory
• fetchSessions now takes an optional byProject parameter to decouple fetch strategy from worktree detection
• SDK types and generated bindings for new endpoints

Test plan

• Open desktop app with multiple sessions across different directories
• Verify directory tree shows correct session counts per directory
• Verify Show sessions (N) matches actual sessions returned when expanded
• Switch between directories and projects view modes
• Verify pagination works for directories with many sessions

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