fix: show tool result images outside event groups

[kanazawahere]

Summary

• treat native image parts as readable chat content so tool-produced images remain visible outside collapsed event groups
• keep the associated tool call/result metadata in the technical EVENTS group
• preserve message metadata when splitting technical and readable parts

Why

Pi Web already renders native ImageContent, but chatGroups.ts classified every image as technical. Once the assistant emitted readable text, the event group collapsed and mobile users had to reopen EVENTS to see a screenshot or QR code produced by a tool.

Validation

• npx vitest run --config vitest.config.ts src/client/src/chatGroups.test.ts src/client/src/components/ChatView.test.ts
• npm run verify — 166 test files, 1117 passed, 2 skipped
• pre-commit npm run verify:staged passed
• also applied to the current released baseline v1.202606.7: 108 test files / 787 tests passed
• canary-tested on a 412px mobile Pi Web session: image visible without opening EVENTS; operator confirmed