Migrate to WXT and Manifest V3; upgrade all dependencies

[joshdick]

Summary

Modernizes tabclip's toolchain and extension platform target, with no intended change to the extension's look, feel, or behavior beyond the bug fixes noted below. Manually verified end-to-end in both Chrome and Firefox.

Bundler: Webpack -> WXT

• WXT is a Vite-based, actively-maintained framework built specifically for cross-browser WebExtensions, and generates the Chrome/Firefox manifest divergence (service_worker vs. scripts background, offscreen permission, etc.) from a single config instead of a hand-rolled multi-entry Webpack config.
• Source restructured into WXT's entrypoints/ convention: src/entrypoints/{background.js, popup/, offscreen/}, with shared logic split out of the old shared.js into src/lib/{prefs,tabs,clipboard-dom,clipboard-offscreen-client,fontawesome-icons}.js.
• ESLint migrated to v10's flat config (eslint.config.js), preserving the existing style rules (tabs, single quotes, no semicolons, unix linebreaks).
• CI moved from CircleCI to GitHub Actions (.github/workflows/ci.yml), using actions/setup-node's built-in npm caching and npm ci.

Manifest V2 -> V3

• browser_action -> action, background.page -> service_worker (Chrome) / scripts (Firefox), MV3 permissions incl. Chrome-only offscreen.
• Chrome's MV3 background is a real service worker with no DOM at all, but the extension's clipboard read/write relies on a hidden contenteditable div and document.execCommand. Firefox's MV3 background is still a DOM-having event page and can keep using that technique directly, but Chrome's keyboard-shortcut copy/paste now delegates to a lazily-created offscreen document (Chrome's documented pattern for clipboard access from a service worker) instead. The popup is unaffected either way, since it always has its own DOM.
• Added the extension's existing AMO-assigned Firefox ID via browser_specific_settings.gecko.id, required for Firefox MV3 submissions so the update isn't mistaken for a new listing.

Bug fixes

• The copy-tabs keyboard shortcut handler destructured PREFERENCE_NAMES.INCLDUE_TITLES (a typo for INCLUDE_TITLES), so the "Include titles" preference was silently ignored when copying via Ctrl+Shift+C (it worked fine from the popup's Copy button).
• browser.extension.getURL (a deprecated WebExtension API) is now browser.runtime.getURL.

Dependency upgrades

• All dependencies upgraded to latest. The deprecated @fortawesome/fontawesome + @fortawesome/fontawesome-free-solid packages are replaced with the current @fortawesome/fontawesome-svg-core + @fortawesome/free-solid-svg-icons, reproducing identical icon rendering via library.add() + dom.watch().
• bootstrap-css-only stays at 4.4.1 (no newer version has ever been published, and jumping to Bootstrap 5 risked visual changes).

Also bumps the extension version to 1.5, now derived from package.json's version field (previously hardcoded separately in the WXT config, out of sync with package.json's long-stale 1.0.0).

Test plan

• npm run lint passes
• npm run build succeeds for both chrome-mv3 and firefox-mv3, manifests diffed field-by-field against the original MV2 manifest
• Popup copy/paste (current/all windows, include titles, background paste, pref persistence) verified in both browsers
• Keyboard shortcuts (Ctrl+Shift+C/V) verified in both browsers, including the Include-titles bugfix and the Chrome offscreen-document clipboard path
• FontAwesome icons render correctly in the popup