feat: add CAD monochrome plot mode

[brownie-cake]

Summary

Integrate the released CAD engine color policy into File Viewer without a CSS filter.

• Adds source/monochrome options, toolbar state, i18n labels, and view-state reporting.
• Keeps the parser-owned CAD source for layer updates and defaults monochrome output to black.
• Preserves Vue 2.6 and Vue 2.7 sample paths.

Related Issue

Closes #213. The same engine-level behavior also covers the monochrome print requirement in #245.

Change Classification

• User-visible UI or rendering change
• Non-visual change
• File-format or renderer behavior
• Public API, package, Worker, WASM, or deployment-path change

Verification

• pnpm --filter @file-viewer/renderer-cad verify:color-mode — Pass.
• pnpm --filter @file-viewer/renderer-cad type-check — Pass.
• pnpm vitest run test/core-operations.spec.ts test/core-registry.spec.ts — Pass (43 tests).
• pnpm audit --registry=https://registry.npmjs.org --audit-level=low — Pass; the documented Vue 2 low advisory remains ignored.
• Real Chromium regression: 4,842 authored-color pixels, 0 colored pixels in monochrome mode, restored source colors, layer toggle, HTML export, and Print — Pass.

Sample / Fixture Evidence

• apps/viewer-demo/public/example/sample.dwg

Visual Evidence

Risk And Compatibility

• Affected packages/formats: @file-viewer/core and the CAD renderer for DWG, DXF, DWF, and DWFX.
• Compatibility or migration risk: older CAD engines without setColorMode() keep the toggle disabled; Vue 2.6 and Vue 2.7 samples remain unchanged.
• Rollback: revert the File Viewer CAD integration and dependency update; source documents remain unmodified.

Checklist

• I added or updated focused automated coverage, or explained why it is not needed.
• I updated user-facing documentation or release notes when behavior or API changed, or marked them not applicable.
• I verified offline/private-deployment paths when changing Worker, WASM, fonts, vendor assets, or URLs.
• I did not commit secrets, customer files, private samples, generated caches, or unrelated changes.

[wybaby168]

Not merging as-is, and I would rather put the reason on record than close it quietly. packages/renderers/cad pins @flyfish-dev/cad-viewer@0.8.0, and that published engine has no setColorMode, so cad.monochrome would have nothing to call at runtime. The blocker is the engine version, not your design.
Once a cad-viewer release exposes a colour-mode API, a rebased PR against it, dropping the public-ci.yml edit and adding a rendered before and after of the same DWG, is welcome. Leaving this open.