fix(nexus): make image generation route stable and executable

[smackgg]

Summary

• expose only executable Nexus image-and-text models, excluding auto and image-only routes that cannot run through the metered Chat Completions path
• prepare one bounded image route during tools/list and reuse its exact Data Token, Provider, and model catalog in the following image.generate call
• prevent tools/call from repeating authorization, Provider, or model discovery after the Host has entered its external-call safety boundary
• serialize credential-store commits and reject delayed access/Data Token refreshes after sign-out or reauthorization
• preserve at-most-once operation replay and invalidate stale route/catalog work
• retain the Storyboard launcher layout fix at the packaged viewport
• release Nexus Service 0.3.15, Nexus companion 0.3.12, and Storyboard Studio 0.1.1

Root cause

Nexus image generation discovered the route three times: model listing loaded token/Provider/models, image.generate loaded models again, and the completion call loaded token/Provider again. A transient discovery failure after tools/call began was still pre-billable inside Nexus, but the Host correctly had to classify the non-LRO external result as unknown and lock the card.

Verification

• bun run check on the final branch after merging latest main
• Nexus tests: 34 passed, including delayed refresh × sign-out/reauthorization races
• compiled darwin-arm64 companion stdio E2E: initialize → tools/list → image.generate
• exact request counts: token / Provider / models / completion = 1 / 1 / 1 / 1
• generated 77-byte PNG matched byte-for-byte
• stdout/stderr contained no refresh/access/Data Token, prompt, output directory, or native temp path
• current 767714f darwin-arm64 companion completed one real production Nexus call with the existing user authorization: google/gemini-2.5-flash-image, one visually verified 1024×1024 PNG (155,087 bytes), and zero stderr lines

[smackgg]

@youking-lib @superche @fearclear @JPJeePee 麻烦任一位 review/approve 最新 head 767714f。CI 已完整通过；并且刚用该 head 编译出的 darwin-arm64 companion、现有生产 Nexus 授权和实时模型目录完成了一次真实调用：google/gemini-2.5-flash-image 返回 1 张经肉眼确认的 1024×1024 PNG（155,087 bytes），stderr 0。不是 fake gateway。修复保留 #57 的 USD budget 展示，并消除了 Host 接受任务后重复 token/Provider/model discovery 导致的 unknown 锁卡。