build: prepare Studio .12 Windows preview

[PurplePrintAI]

Summary

• Prepare PurplePrint Studio 1.10.0-subscription.12 as a Windows x64 preview candidate after the merged image-scene and chat-persistence PRs.
• Update package.json and package-lock.json together, and sync the Nix dependency hash computed by CI.
• Move the two fork changes into a .12 candidate changelog section, correct stale README wording, and add draft Windows release notes with explicit publication gates.

Verification

• npm ci succeeded (local Node/npm are slightly older than the pinned engine versions; CI uses the pinned versions).

• Release-version Vitest tests, docs check, i18n check and lint passed. Lint has 13 pre-existing warnings.

• Full CI (https://github.com/PurplePrintAI/openscreen-subscription/actions/runs/36513534474) and clean Windows packaging (https://github.com/PurplePrintAI/openscreen-subscription/actions/runs/36513534491) passed for package PR head feff6d2. The PR checkout merge commit was 1631227. The unsigned installer is 245,994,048 bytes; SHA-256 B7611D41E5B0C607B8400958DE1ED12CC783096DC9110ED7AF0D538030A33C3A matches the CI manifest and checksum list, and SHA-512/size match latest.yml. The only later commit updates this draft note; it does not change package inputs.

• Owner-tested docs-only PR head e46f551 also passed full CI (https://github.com/PurplePrintAI/openscreen-subscription/actions/runs/36514529135) and a second Windows package (https://github.com/PurplePrintAI/openscreen-subscription/actions/runs/36514529127). Its distinct, unsigned installer is 245,994,029 bytes, SHA-256 AB32A87267E44A924FCD7E6036D6B28F1CAB6CEF7ADB4850ADD6EFF2181BAE2A. Both artifacts have their own matching checksum/validation files; do not mix them.

• The owner installed the e46f551 .12 candidate after a verified active-profile backup: 5,637 files, 8 projects, 1 chat history and 2 generated-scene files. All 13 important hashes survived installation and QA project launch. The editor displayed the saved two-message conversation and five-second image scene; the owner reported MP4 export success. No additional AI generation was used, and this agent did not decode the exported file.

• Latest documentation-only head 1580509 passed full CI (https://github.com/PurplePrintAI/openscreen-subscription/actions/runs/36517582781) and another Windows package (https://github.com/PurplePrintAI/openscreen-subscription/actions/runs/36517582766). That unsigned artifact was not installed: 245,994,010 bytes, SHA-256 AF5B2EABAA2B9FBC82A970E0E839EDE59E6B6D4C073B9DB2EDE557890D87D099. Only this release-notes draft changed since the owner-tested e46f551 head; each artifact retains its own checksum set.

Scope and risks

• This PR does not create a tag, GitHub Release, or public download.
• Windows installer remains unsigned. There is no .12 Mac/Linux installer; Mac real-world validation, signing and notarization remain open.
• Codex CLI is user-installed, image generation consumes subscription limits, and local conversation files are plaintext. Previously lost chats cannot be recovered.
• Public publication requires separate owner approval after the draft checklist is complete.