ui: port findings panel to new design

[Moriz82]

Summary

• Rewrites FindingsPanel.tsx to the new-UI two-pane layout: severity summary stats, searchable list with chip filters, and a detail pane (target / evidence / remediation / actions).
• Wires to real backend (/api/findings?limit=5000); severity counts are derived client-side from the list. No mock data.
• Adds components/icons.tsx + components/Severity.tsx primitives, styles/screens.css with tokens and findings selectors.
• Mounts the Findings tab in the Operations nav group (it was imported but never rendered). Adds onOpenInGraph prop + gossamer:navigate / gossamer:open-in-graph window events so panels can flip tabs without prop drilling.

Action wiring

• Rerun scan → dispatches gossamer:navigate to Operations (rescans need a scanner + target picked in that flow).
• Export report → client-side JSON download of findings + counts.
• Open in graph → calls onOpenInGraph(id) or falls back to gossamer:open-in-graph event.
• Copy cURL → builds curl -sk -i '<target>' from the finding's target URL, copies to clipboard.
• Mark as triaged / Suppress → disabled with "coming soon" tooltip (no backend mutation endpoint exists; per spec we don't fake the state).

Test plan

• npx tsc --noEmit clean for all touched files (pre-existing errors in GraphPanel/OperationsPanel are unchanged).
• npm run build succeeds.
• Playwright E2E: loaded /, navigated to Findings, toggled a severity chip, clicked a finding row. Detail pane rendered correctly with target / evidence / remediation / actions. Zero console errors.

Screenshot (1 real finding from a live crawl): Findings list + detail pane render; target extracted from the |-delimited finding key when matched_at/url are absent.

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