Add aquaticBackground/ module with OceanScene (unified gradient, light rays,
sea floor, corals, seaweed, decorations), plus independent overlay layers
(BubbleStream, FishSchool, JellyfishDrift, EventOverlay, EdgeFade). Includes
event scheduling engine with 4 frequency tiers (minutes/hours/days/months)
and 20 random events with localStorage persistence.
Add shared CodeBlock component with copy-to-clipboard button, terminal-matched
monospace font (Consolas), and proper line-height/letter-spacing. Refactor
EditCard, WriteCard, TaskCard, and ToolResultBlock to use CodeBlock. Fix
markdown code block alignment to match terminal rendering.
