Make sidebar resizable

[jpenilla]

Allows resizing the sidebar while maintaining the same general layout and sidebar left/right positioning ability.

Known issue:
When opening a new window with an existing layout cookie for a different sized window, the absolute sidebar size will change as only proportions are persisted. There is a TODO in the code with more details, but fixing this properly needs changes in paneforge to support setting a size in px for SSR.

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[Copilot]

Pull Request Overview

This PR adds resizable sidebar functionality to the diff viewer using the paneforge library. Users can now manually adjust the sidebar width by dragging a resize handle, with the layout state persisted in cookies across sessions.

• Integrates paneforge library for split pane layout with resizable sidebar
• Implements layout state persistence via cookies for sidebar width
• Adds keyboard shortcut (Ctrl/Cmd+B) to toggle sidebar visibility
• Refactors sidebar positioning logic to work within the paneforge layout system

Reviewed Changes

Copilot reviewed 13 out of 14 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
web/package.json	Adds [email protected] dependency for resizable panes
web/src/lib/layout.svelte.ts	New LayoutState class managing sidebar width, collapse state, and layout persistence
web/src/lib/util.ts	Adds setCookie and clearCookie helper functions for layout persistence
web/src/lib/keybinds.svelte.ts	Renames getModifierKey to formatModifierKey (private) and adds formatModifierBind for consistent keybind formatting
web/src/lib/global-options.svelte.ts	Uses new setCookie utility function instead of inline cookie setting
web/src/lib/diff-viewer.svelte.ts	Integrates LayoutState and registers Ctrl/Cmd+B keybind for sidebar toggle
web/src/routes/+layout.svelte	Moves GlobalOptions.init from page to layout for earlier initialization
web/src/routes/+page.server.ts	New server load function to retrieve persisted layout state from cookies
web/src/routes/+page.svelte	Replaces manual sidebar positioning with PaneGroup/Pane/PaneResizer components
web/src/routes/Sidebar.svelte	Simplifies sidebar component by removing positioning logic and click-outside handling
web/src/routes/FileHeader.svelte	Updates to use layoutState.sidebarCollapsed
web/src/lib/components/SidebarToggle.svelte	Moved from routes/ and updated to use layoutState.toggleSidebar()
web/src/lib/components/menu-bar/MenuBar.svelte	Adds "Toggle Sidebar" and "Reset Layout" menu items, includes SidebarToggle in menubar

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