feat: implement VS Code-style activity bar sidebar (#920)

Replace the text-label sidebar with a two-column layout: a 48px icon
activity bar (always visible) and a collapsible content panel. Uses
existing shadcn/ui sidebar primitives with collapsible="icon" mode.

New components:
- ActivityBar: vertical icon strip with logo, 5 nav icons, settings
- ActivityBarItem: icon button with left-border active indicator + tooltip
- SidePanel: conditional content renderer for active section
- ThreadsPanel: extracted virtual-scroll thread list
- ProjectsPanel: extracted project list with create button
- SchedulesPanel: extracted schedule execution list

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: ryaneggz <kre8mymedia@gmail.com>

Author: ryaneggz