fix: add error logging to empty catch blocks

[gugli4ifenix-design]

Problem

6 empty catch blocks across 5 files silently swallow errors:

• httpClient.ts (×2): Response parsing errors are invisible — if the server returns malformed JSON, data silently stays undefined
• mermaidRenderer.tsx: Mermaid diagram render failures are hidden
• editor/index.tsx (×2): Editor errors are suppressed
• ShowText/index.tsx: Content parse errors are silenced

Fix

Add console.warn to all empty catch blocks:

// Before
} catch (error) {}

// After  
} catch (error) { console.warn("[PandaWiki] Response parse error:", error); }

• 5 files, 6 catches updated
• Behavior unchanged — errors are caught, not thrown
• Debugging improved — errors are now visible in console

Found during a static analysis sweep of 97 popular open source projects.