Make the slot machine its own sidebar entry; rework its visuals

Moves Slot Machine into the sidebar's account section (alongside RZ
Shop/Settings/Log out) instead of being reachable only from the shop -
removes the shop's promo banner and the screen's "Back to Shop" link
accordingly.

Also closes a chunk of the visual gap against the mockup: a proper
cabinet-style panel (arched top corners, glowing border), a light
"screen" panel holding a real 3x3 reel grid instead of 3 single icons,
a decorative lever, an "RZ" marquee, and the cost/spin/auto-spin
controls each in their own bordered box. Added two new vendored icons
(gem, flask-conical) for reel variety and reward-list icons.

assets/icons/lucide/flask-conical.svg

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