Bump tailwindcss from 2.2.19 to 4.2.1

[n-lark]

Bump tailwindcss from 2.2.19 to 4.2.1 (#6782)

Major version bump spanning two majors (2 → 3 → 4). The branch is currently blocked on merge conflicts and has 6 failing CI checks:

• Tests / Backend tests (pull_request)
• Tests / Notify on failure (pull_request)
• Tests / Postgres tests (pull_request)
• Tests / Tests summary (pull_request)
• Tests / UI EE tests (pull_request)
• Tests / UI OS tests (pull_request)

Coordinate with the related work in FlowFuse/flowfuse#6286, which has also been left to rot and has its own conflicts. Tailwind 3 → 4 is a significant migration (new engine, config format changes), so this needs dedicated time, not a 30-minute slot. Resolve conflicts first, then work through the failing suites — many will likely be downstream of the Tailwind config/CSS changes.

[n-lark]

Relates to #6286

[n-lark]

Tailwind 2.2.19 → 4.2.4 Upgrade (#7141)

What changed

• Bumped tailwindcss to 4.2.4, added @tailwindcss/postcss plugin, switched index.css to @import "tailwindcss"; @config "...".
• Reshaped tailwind.config.js to v4 (dropped mode: 'jit', purge wrapper, variants; lifted content/safelist to top level).
• Unrolled forge-button-* @apply chains (v4 no longer expands custom classes via @apply).
• Ran npx @tailwindcss/upgrade (mechanical renames across 83 .vue files).
• Manual sweep for what the tool missed: outline-none → outline-hidden, ring-opacity-N → slash syntax, placeholder-gray-500 → placeholder:text-gray-500, border-opacity-0 → border-transparent.
• Restored v3 defaults: universal border-color: var(--color-gray-200, currentColor) in @layer base; explicit border-gray-200 on RightDrawer; hr { color: $ff-grey-200 }.
• Wrapped global h1–h4 SCSS rules in @layer base (v4's stricter cascade).
• Applied the four official v4 scale renames: shadow-sm/shadow → shadow-xs/shadow-sm, rounded-sm/rounded → rounded-xs/rounded-sm.

Decisions

Decision	Why
Restart fresh, do not salvage upgrade-tailwind branch	2,007 commits stale, only reached 4.0.0; rebase would conflict with major dep bumps on main (fastify@4→5, uuid@9→14). Kept as reference only.
Minimal v4 JS config, not CSS-first	Keeps the 45-item safelist working unchanged; isolates version bump from a separable refactor.
Unrolled @apply chains before running the upgrade tool	The tool compiles CSS during its template pass and errors on @apply forge-button; (v4 dropped custom-class expansion).
No ring default override	Every ring-* callsite specifies explicit width and color, so v4's 1px-currentColor default never applies.
Reverted full shadow/rounded scale shift; kept only the four official renames	v3 and v4 ship identical CSS values for shadow-md/lg/xl and rounded-md/lg/xl/full.