refac: Extract shared script utilities into dev-tools

[simonrelet]

Summary

• Extract duplicated script utilities into @animeaux/dev-tools: all three apps copied getLengthValuePx, getBreakpoints, getSpacing, safelyReadFile, and relativeToCwd verbatim across their scripts/ directories.
• Move oneAtTheTime from show's local scripts/shared.ts so it is available to any app.

Context

Each app's generate-theme.ts script and sprite generators duplicated the same helper functions with no meaningful variation. Centralising them in dev-tools is a natural fit: every app already depends on it, and dev-tools was always intended to host runtime utilities alongside its config exports.

The shared utilities are pure functions with no brand or app-specific logic — only the getColors / getFonts blocks remain app-specific, as intended.

Changes

• Add libs/dev-tools/src/ with one file per utility: format-breakpoints.ts, format-spacing.ts, get-length-value-px.ts, one-at-the-time.ts, relative-to-cwd.ts, safely-read-file.ts, and a barrel index.ts.
• Expose all utilities under the "." export so callers use import { … } from "@animeaux/dev-tools".
• Rename code-generation helpers from getBreakpointsCode / getSpacingCode to formatBreakpoints / formatSpacing for consistency with the format* convention.
• Add build infrastructure to dev-tools: TypeScript compilation, tsc-alias for #i/* path aliases, build:src / build:paths scripts, and postbuild: pnpm sync-workspace.
• Remove scripts/shared.ts from admin, show, and website (now empty or superseded); update all script imports to @animeaux/dev-tools.

Risks and mitigations

• N/A — pure refactor with no behaviour change. The generated src/generated/theme.ts output in each app is identical before and after.

Checklist

• Added in-code documentation (wherever needed).
• Ran the linter to ensure style guidelines were followed.
• Reviewed the changes for regressions.
• Manually tested the changed behaviour.