feat: regex-default

[bmamlin]

Implements feature request #647 to allow default to regex mode.

• Adds an extension options page with a "Enable Regex search mode by default" toggle
• When enabled, the regex mode toggle in the find bar starts active on every page load
• Settings are persisted via chrome.storage.sync and auto-saved on change
• Also fixes a build bug where the PostCSS plugin for Tailwind v4 was referenced incorrectly

Extension options page (src/pages/options/): A new React-based options page is registered in the manifest and accessible from the extension's details page. It uses a two-column layout to display the setting label and control, scaling cleanly for future options.

Content script startup (src/pages/content/index.tsx): On initialization, the content script reads defaultUseRegularExpression from chrome.storage.sync and dispatches ToggleShouldUseRegularExpression before mounting React, so the find bar always starts in the correct state.

Build fix (scripts/build.ts): The PostCSS configuration was referencing tailwindcss directly as a plugin, which is not supported in Tailwind CSS v4. Updated to use @tailwindcss/postcss to match postcss.config.cjs.

Closes #647