Harden onion boot flow and deepen site surfaces

Add persistent onion key backup and restore, improve startup resilience, and flesh out the major site verticals with richer navigation, search coverage, and operator documentation.

Made-with: Cursor

lib/placeholderOnion.ts

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+/** Fictional v3-style placeholder for all simulated .onion URLs — replace when curating for class. */
+export const PLACEHOLDER_ONION_HOST = "111111111111111111111.onion";
+export const PLACEHOLDER_ONION_URL = `http://${PLACEHOLDER_ONION_HOST}`;