Use bun as much as possible

[agarny]

Fixes #433

[Copilot]

Pull request overview

This PR updates the repo to rely on Bun (runtime + package manager) more consistently across local scripts, package scripts, documentation, and GitHub Actions, aligning with Issue #433.

Changes:

• Add Bun-based helper scripts for version bumping, dependency updates, and copying index.d.ts in the renderer package.
• Update root + renderer package.json scripts/docs to prefer bun run --cwd ... and Bun-driven workflows (including adding engines.bun).
• Update CI/CD workflows to install Bun directly (and remove general Node setup where it’s no longer required).

Reviewed changes

Copilot reviewed 9 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/renderer/scripts/version.new.js	Adds a Bun script to compute and apply a date-based version bump across root + renderer package.json.
src/renderer/scripts/dependencies.update.js	Adds a Bun script to run the existing dependency refresh flow for both root and renderer directories.
src/renderer/scripts/copy.indexdts.js	Replaces cp with a cross-platform Bun/Node-fs copy for index.d.ts into dist/.
src/renderer/package.json	Adds engines.bun, updates scripts to use the new Bun helpers, and bumps version/deps.
src/renderer/bun.lock	Lockfile updates corresponding to dependency bumps.
src/renderer/BUILD.md	Updates renderer build prerequisites/instructions to Bun-only.
package.json	Adds engines.bun, routes web scripts through renderer via --cwd, bumps version/deps.
bun.lock	Lockfile updates corresponding to dependency bumps.
BUILD.md	Updates root build prerequisites/instructions to Bun-first.
.github/workflows/ci.yml	Removes general Node setup and standardizes on Bun install (incl. Windows ARM handling).
.github/workflows/cddev.yml	Removes Node-based Bun install and uses oven-sh/setup-bun.
.github/workflows/cd.yml	Removes general Node setup from non-npm jobs; retains Node where npm publishing requires it.

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