Add mise-driven Chromium minimum and document browser support

[sm17p]

Summary

• Chromium minimum: minimum_chrome_version is set from WXT_MIN_CHROME_VERSION in wxt.config.ts via import.meta.env. The value is not hardcoded in config; supply it through mise.toml [env] or a local .env.
• CI / local parity: mise.toml defines Node and [env] (including WXT_MIN_CHROME_VERSION). Workflows use jdx/mise-action so GitHub Actions load the same toolchain and env.
• Documentation: README and AGENTS describe minimum browser support and the Firefox policy (Firefox strict_min_version stays literal until Firefox can be exercised in automation).
• Skills: WXT and e2e Playwright Cursor skills updated for mise and manifest env.

Other changes

• wxt-env.d.ts: ImportMetaEnv includes WXT_MIN_CHROME_VERSION.
• Small storage / UI store / app.config updates; e2e fixture server tweak; remove demo Playwright tests-examples spec; minor CHANGELOG edit.

Test plan

• pnpm check and pnpm lint
• pnpm build with mise active (or WXT_MIN_CHROME_VERSION set in the environment)

Closes #35

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