refactor: rename the codev-proxy surface to "backend"

[quickbeard]

Summary

Renames the codev-proxy concept to backend across the codebase — identifiers, comments, log labels, and error text — per the agreed scope. ("proxy url → backend url" was already settled when the hidden custom-proxy-URL step was removed in #163; the constant is BACKEND_URL.)

Changes

Code identifiers

• src/lib/proxy.ts → src/lib/backend.ts (and tests/lib/proxy.test.ts → tests/lib/backend.test.ts)
• All @/lib/proxy.js imports → @/lib/backend.js (6 source files)
• Test namespace import * as proxy → backend and every vi.spyOn(proxy, …) → vi.spyOn(backend, …) (5 test files)
• PROXY_TIMEOUT_MS → BACKEND_TIMEOUT_MS

Strings (per the "labels + error text" scope decision)

• Log endpoint labels "proxy.auth-exchange" / "proxy.config" / "proxy.supabase-exchange" → "backend.*"
• Thrown error prefixes "Proxy /… failed" → "Backend /… failed" (+ all tests pinning them)

Comments

• Reworded codev-proxy/proxy references to "backend" (log.ts, upload.ts, supabase.ts, auth.ts, AGENTS.md, a couple of test descriptions)
• Fixed a stale upload.ts comment that still referenced the removed custom proxy-URL

Intentionally NOT changed

• The live route ${BASE_URL}/codev-proxy and every url.includes("/codev-proxy/…") test matcher / fixture URL — that's the deployed server contract.
• Generic/corporate network-proxy mentions unrelated to our backend: Login.tsx, vscode.ts, vscode.test.ts, and the Continue-marketplace "Proxy returned 502" test in InstallApp.test.tsx.

Validation

• pnpm fix — clean (Biome reordered imports in the renamed files)
• pnpm typecheck — clean
• pnpm test — 720 passed / 50 files
• pnpm build && node dist/index.js --version → 0.3.7

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