Add mode switch between Chat & Agent to the desktop sidebar

[marksftw]

Summary

Replace the standalone Agent Mode item under New Chat with a two-option mode switch directly below the Maple logo:

[ MessageCircle Chat | Bot Agent ]

The control changes which workspace the sidebar and main content show. Chat and Agent retain separate histories.

Requirements

• Place the switch immediately below the Maple logo.
• Use two equal-width segments labeled Chat and Agent.
• Show a Lucide MessageCircle icon for Chat and Bot for Agent.
• Keep both text labels visible; icons are never used alone.
• Use 16 px outline icons with consistent stroke weight.
• Do not reuse the SquarePen icon used by New Chat.
• Make the selected mode visually clear with a filled surface and stronger text/icon treatment; do not rely on color alone.
• Match Maple’s existing quiet sidebar styling.

Behavior

• Selecting the inactive segment switches modes immediately.
• Selecting the active segment is a no-op.
• Switching modes does not create, reset, or delete a chat or task.
• Each mode shows its own history and restores its most recently viewed item when available.
• Keep the sidebar open during the transition.
• Remove the existing standalone Agent Mode menu action.

Accessibility

• Expose the control as one named mode selector with two mutually exclusive options.
• The current selection must be announced to assistive technology.
• Both options must be reachable and operable by keyboard.
• Provide visible focus states and tooltips that clarify the destinations if needed.

Acceptance criteria

• The switch appears below the Maple logo in both modes.
• Chat is selected in Chat mode; Agent is selected in Agent mode.
• The correct icon and persistent label appear for each option.
• Switching changes the visible workspace without mixing histories or creating a new item.
• The prior Agent Mode menu action is gone.
• Keyboard and screen-reader behavior communicates the selected mode correctly.

Out of scope

• Renaming Agent-mode chats and sessions to tasks; that is tracked in a separate issue (Rename Agent chats and sessions to tasks in user-facing UI #639).
• Changes to the underlying chat/task data model.