Mission Control: switch between Tasks and Loops

[pallaoro]

Adds a Tasks / Loops switch to Mission Control, so the grid tiles either the sidebar's tasks or the sessions owned by the project's loops, never both at once. Before this, loop sessions were mixed in with task tiles.

• The switch sits right after the view tabs (left of the centered search, which covered it when placed beside the layout buttons at a 1400px window). It shows only while the grid is on screen.
• Loops mode orders tiles like the sidebar's LOOPS list; Tasks mode follows TASKS as before.
• Both modes share the locked tile order (ateam.mcOrder.<project>), so a drag in one mode keeps the other's arrangement.
• Switching modes remounts the grid (same reason as Mission Control: stop showing the previous project's terminals #271), so the previous mode's tiles never linger.
• The choice is app-wide (ateam.mcMode), like the lock.

Verified in the dev app over CDP: the switch toggles, the empty state follows the mode, and the choice persists. Typecheck passes; Biome error count on App.tsx is unchanged from main.

Not covered: clicking a loop in the sidebar while in Tasks mode opens its task full-width rather than flipping modes; the mobile Mission Control screen is unchanged.