Migrate frontend deploy from bulletin-deploy to polkadot-app-deploy

[illegalcall]

What

Replaces the retired bulletin-deploy with its successor polkadot-app-deploy (npm: @parity/polkadot-app-deploy) in the frontend deploy path, and updates DEPLOY_DOC.md step 7 accordingly. Resolves the "Pending W3S environment support" caveat: the new tool ships the W3S/Summit environment.

Notes for review

• Env id is summit, not w3s. Verified against the published v0.10.0: --list-environments shows paseo-next-v2 and summit; --env w3s fails with Unknown environment 'w3s'. CDM's own chain preset is still named w3s (known_chains.ts), which is a separate namespace; the doc spells this out.
• Signer flag changed semantics: the new tool's deploy path takes --mnemonic (bare mnemonic) instead of --suri. A //Alice-style SURI no longer works; derivation goes through the tool's separate --derivation-path. The CI secret feeding CDM_DEPLOY_SURI is named MNEMONIC, so this should be compatible, but worth confirming.
• Env var renames, no back-compat: BULLETIN_DEPLOY_ENV -> APP_DEPLOY_ENV, SKIP_BULLETIN_INSTALL -> SKIP_APP_DEPLOY_INSTALL. The only consumer (DEPLOY_DOC.md) is updated in this PR; .github/workflows/deploy-frontend.yml never set either, so it is unaffected.
• Merging arms a live CI deploy: deploy-frontend.yml runs this script on every push to main, so the first post-merge push redeploys contracts.dot (on paseo-next-v2) via the new tool. Recommend watching that run, or triggering it deliberately via workflow_dispatch. The runner has no IPFS Kubo; the tool falls back to pure-JS merkleization, but passing --js-merkle explicitly in CI is cheap determinism if preferred.

Verification

Ran the script end to end with the real frontend build and a PATH shim capturing the final invocation (to avoid publishing to the live contracts.dot): argv is exactly --env summit --mnemonic <mnemonic> <dist> contracts.dot plus passthrough args. Installed the real npm package in a temp prefix to confirm the binary name, flags, and environments table. Probed: no-args usage (exit 1), default env without APP_DEPLOY_ENV, and legacy BULLETIN_DEPLOY_ENV being ignored. Swept the repo for any remaining bulletin-deploy references: none outside the two files changed here.