Add Brave web search with 5p charge per query

[Copilot]

Adds a /search endpoint that combines local indexed content with Brave web search results, displayed in parallel sections. Charges 5 credits (5p) per query via the existing wallet quota system.

New: search package

• Handler runs data.Search (local) and Brave Search API in parallel via goroutines
• Results rendered in two sections: Local Results / On the Web
• Requires authentication; quota checked before search, consumed after
• Gracefully degrades: Brave errors surface as "Web search unavailable" without failing the local results

Wallet

• New OpWebSearch = "web_search" operation constant
• CostWebSearch = getEnvInt("CREDIT_COST_SEARCH", 5) — overridable via env var
• Wired into GetOperationCost; plans page updated to reflect 5p cost

Navigation

• Search input added to the top of the sidebar nav (submits to /search?q=)
• CSS added for #nav-search to style the inline input

GET /search?q=bitcoin
→ local: data.Search("bitcoin", 10)          // in parallel
→ web:   Brave API X-Subscription-Token: $BRAVE_API_KEY
→ deduct 5 credits from wallet on success

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