Add a collapsible and resizable sidebar

[kiranfin]

User Story

As a frequent user, I want to resize or collapse the sidebar, so that I can dedicate more space to my current task.

Context & Motivation

The sidebar currently occupies a fixed amount of space with no way to shrink it.

Acceptance Criteria

• The desktop sidebar can collapse to an icon-only state.
• The desktop sidebar width can be adjusted within sensible minimum and maximum values.
• The selected width and collapsed state persist between sessions.
• Every icon-only navigation item has an accessible tooltip and label.
• Mobile sidebar behavior remains unchanged.

Technical Notes

Confirmed in SideBar.tsx: the desktop sidebar has a hardcoded fixed width (w-[286px]) with no collapsed/icon-only variant, no resize handling, and no localStorage/sessionStorage persistence. The only existing state toggles the mobile drawer and is unrelated to this feature.

Related: a keyboard shortcut to toggle the sidebar is covered by the separate app-wide keyboard shortcuts issue, since that request is broader than just this component.