fix(web-app-ai-sensitive-data-scanner): render scan findings instead of raw json

[LukasHirt]

Summary

• The LLM proxy requests response_format: { type: 'json_object' }, but in practice the model frequently wraps its JSON response in a markdown code fence (```json ... ```) anyway. parseLlmResponse in useScanner.ts called JSON.parse directly on the raw content, so the fenced response failed to parse and the entire fenced blob (including the ``` fences) fell through to the narrative fallback, which the component renders verbatim as preformatted text — producing the "raw JSON dumped in a code fence" bug shown in the issue screenshot.
• Fixed parseLlmResponse to:
  1. Strip a leading/trailing markdown code fence (```json ... ``` or bare ``` ... ```) before attempting to parse.
  2. If that still fails, fall back to extracting the substring between the first { and the last } (handles cases where the model adds leading/trailing prose around the JSON object).
  3. Only fall back to storing the raw response as narrative if none of the above yield a valid { findings: [...] } payload — preserving the existing plain-text fallback behavior for genuinely non-JSON responses.
• No changes to the component's rendering logic — ScanResultsModal.vue's existing findings-list rendering (icon + category label + excerpt) was already correct; the bug was purely in parsing.

Test plan

• Added packages/web-app-ai-sensitive-data-scanner/tests/unit/composables/useScanner.spec.ts covering: plain JSON, ```json fenced JSON, bare ``` fenced JSON, JSON with surrounding prose, and genuine non-JSON prose (narrative fallback).
• pnpm --filter web-app-ai-sensitive-data-scanner test:unit — all tests pass (24/24, including existing ScanResultModal.spec.ts).
• pnpm --filter web-app-ai-sensitive-data-scanner check:types — passes.
• pnpm --filter web-app-ai-sensitive-data-scanner lint — passes.

Closes #531

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[kw-security]

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