feat: Modular aquatic background system and shared CodeBlock component

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.

frontend/src/components/transcript-debug/index.ts

@@ -11,4 +11,5 @@ export { default as SystemMessage } from './SystemMessage.vue'
 export { default as UserInput } from './UserInput.vue'
 export { default as PermissionApproval } from './PermissionApproval.vue'
 export { default as PlanApproval } from './PlanApproval.vue'
-export { default as BackgroundPixelArt } from './BackgroundPixelArt.vue'
+export { default as CodeBlock } from './CodeBlock.vue'
+export { AquaticBackground } from './aquaticBackground'