fix(deps): upgrade tailwindcss to v4 and eslint to v10

[TartanLeGrand]

Why

#204 (Snyk) bumps tailwindcss 3 to 4 and eslint 9 to 10 in package.json only:

• package-lock.json is not updated, so npm ci fails on it (lockfile out of sync);
• Tailwind 4 drops tailwind.config.js + the tailwindcss PostCSS plugin, so the build breaks without a migration;
• ESLint 10 is refused by the installed eslint-plugin-react-hooks 5 (peer range stops at 9).

The advisory it targets (braces, uncontrolled recursion) only reaches us through Tailwind 3 (chokidar, micromatch) at build time, and braces has no patched release: dropping Tailwind 3 is the fix. urijs is not in the dependency tree.

This PR does the full upgrade and supersedes #204.

What

Tailwind 4, migrated with @tailwindcss/upgrade, then compared page by page with main:

• configuration moved to @theme in src/index.css; PostCSS uses @tailwindcss/postcss (autoprefixer is built in);
• templates renamed to the v4 utilities (shadow-sm to shadow-xs, rounded to rounded-sm, outline-none to outline-hidden, ...);
• dark mode overrides of the legacy gray classes kept as plain CSS. Tailwind 3 also applied them to variants (dark:text-gray-300, dark:hover:bg-gray-800, ...) because they sat in @layer utilities; v4 does not, so the exact selector list v3 generated is written out;
• FontAwesome CSS imported into Tailwind's base layer (config.autoAddCss = false): injected unlayered at runtime, it beats every v4 utility and icons sized with w-8 h-8 collapsed to 1em;
• v3 absolute line heights and placeholder color restored;
• two rewrites of the upgrade tool reverted: a color token argument a('outline', ...) turned into outline-solid (caught by tsc), and the active nav icon text-[#E8580A] swapped for icon-gradient-yellow, which turns yellow in dark mode.

ESLint 10 with @eslint/js 10 and eslint-plugin-react-hooks 7. Its recommended preset now bundles the React Compiler rules (33 new errors); the two classic rules are configured explicitly so the lint scope is unchanged. One new no-useless-assignment hit fixed in ThemeContext.tsx.

Checks

• npm ci and npm audit: 0 vulnerabilities.
• npm run build and npm run build:static: green.
• tsc --noEmit: 122 errors, same as main. eslint .: 78 errors / 165 warnings, same as main.
• Visual comparison of the static build against main: 11 pages (dashboard, timeline, calendar, catalog, drifts, RPA, locks, links, insights, docs, theme test) in light and dark mode, pixel diff. Remaining differences are the v4 default palette (OKLCH, slightly more saturated: banners, a few links and chart bars).

Notes

• feat(web): login page, auth context and admin pages #201 also touches web/package.json and a few templates; whichever merges second needs a rebase.
• Adopting the React Compiler lint rules is left for a separate change.

Supersedes #204