docs(deploy): add the missing PC-web copy step to the release runbook

The release steps only covered the APK + version.json, so the web build
served at sea.lucas-ai.cc/ (express.static of ~/.www/sea-app) silently
fell behind — phones updated, browsers did not. Document the dist copy
as an explicit numbered step with an overwrite-only warning (sea-app
holds extra assets like fish/ that dist does not contain).

Constraint: never wipe ~/.www/sea-app — overwrite only.
Confidence: high
Scope-risk: local
Reversibility: trivial
Directive: none
Tested: followed the new step for 0.71.0; / now serves the new bundle.
Not-tested: n/a (docs only)
Related: proxy/src/index.ts static mounts

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