Split view, chat organization & UI overhaul

[OpenSource03]

Summary

WIP — This PR is a work in progress. Not ready for review yet.

• Split view system — up to 4 concurrent chat panes with drag-to-split from sidebar, per-pane tool drawers, resizable handles, and independent engine state per pane
• Chat organization — folders, pinning, branch-grouped sidebar sections with drag-and-drop session management and folder CRUD via IPC
• Mac background effects — switchable liquid glass / vibrancy / off with runtime capability detection and graceful fallback
• Tool icon system — per-tool colored glyphs (ToolGlyph component) with show/hide and colored/monochrome appearance settings
• ANSI color rendering — full SGR escape sequence parser rendering colored terminal output in bash tool results
• Assistant turn dividers — "Worked for Xm Ys" duration labels between agent turns based on message timestamps
• Diff stats — +/- line count badges on Edit/Write tool cards extracted from unified diffs and structured patches
• Browser panel — per-tab color scheme control, DevTools open/close state tracking
• Unified engine picker — merged model/config/effort dropdowns into single dropdown alongside agent switching
• Session record helpers — extracted toChatSession() and buildPersistedSession() to eliminate inline duplication
• UI polish — TodoPanel redesign with progress bar, custom InlineSelector replacing ShadCN Select in git panel, chat layout constant extraction, sidebar SessionItem/FolderSection/ProjectSection refinements, extensive CSS updates

Test plan

• Verify split view: drag session from sidebar, drop between panes, resize handles, close pane, dismiss all
• Verify folders: create, rename, delete, move sessions between folders, pin folders
• Verify mac background effects: switch between liquid glass, vibrancy, off in settings
• Verify tool icons: toggle show/hide, colored vs monochrome in appearance settings
• Verify ANSI rendering in bash tool output with colored CLI tools
• Verify turn dividers appear between multi-step agent turns with correct duration
• Verify diff stats badges on Edit/Write tool cards
• Verify browser per-tab color scheme and DevTools state
• Verify engine picker dropdown works for model/agent switching
• Verify sidebar grouping by branch, pinned section, session context menus
• Cross-platform: macOS (glass/vibrancy), Windows (Mica), Linux

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