[cable8mm]

Summary

• Implement the Prefix mode execute flow in src/main.ts, which previously always rendered 'Prefix 모드는 아직 지원하지 않습니다.' regardless of input.
• A non-empty path sends { mode: 'prefix', service, path } to /purge/execute (backend validation in src/purge.ts — V4, V5 — was already implemented and tested, just not reachable from the UI).
• An empty path shows a confirm() dialog naming the selected service before resending with confirmation set, per PRODUCT_SPEC.md V5 / A5 and the ARCHITECTURE.md resolved decision that typing the service name is not required.
• The path input switches to a single row and its placeholder shows a full img.repl.net image URL in Prefix mode, matching the intended "paste a URL, trim it from the right into a prefix" workflow.

Note

.replworks/TASKS.md already marks T-007 ("Purge image URL prefixes") [X], but the operator console never actually exercised this path until now — flagged as a doc/code conflict in an earlier discussion. This PR only wires up the UI; it does not change .replworks/TASKS.md, and a live-Cloudflare E2E run (per T-007's full acceptance criteria) has not been performed against real credentials.

Test plan

• npm run typecheck
• npm test (28 tests pass; backend prefix validation already covered)
• npm run build
• Manually verified in a browser against the local dev server (dummy Cloudflare credentials): service-aware placeholder, single-row input, V4 query-string rejection, and a real (upstream-rejected, since the zone id is a dummy value) prefix purge request round-tripping correctly
• Did not exercise the "confirm empty-path purge" branch end-to-end (blocked by this environment's own safety guard) or run against real Cloudflare credentials

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