build(deps): migrate Tailwind CSS v3.4 → v4

Root-fixes the opacity-modifier seed bug behind R10 S0: the theme colors were
bare `var(--x)` hex with no <alpha-value>, so `bg-accent/40`, `ring-accent/40`,
`border-border/60`, `text-muted/70`, … (~99 sites across 45 files) emitted no
CSS rule at all — latent no-ops. v4 implements the opacity modifier via
`color-mix`, so the modifier now works with the existing hex vars, no per-scheme
token duplication needed.

- `npx @tailwindcss/upgrade`: `@tailwind` directives → `@import "tailwindcss"`,
  JS config → CSS `@theme` (colors, --font-sans, the z-index tokens preserved),
  classname renames (rounded/shadow/blur scale, outline-none→outline-hidden,
  ring→ring-2) across 56 files; PostCSS plugin → @tailwindcss/postcss,
  autoprefixer removed (v4 prefixes for you).
- `@theme` → `@theme inline` (load-bearing): the color vars are redefined at
  runtime under html[data-scheme]/[data-theme]; inline resolves them at the use
  site so per-scheme opacity tints track the active scheme (the shadcn v4 pattern).
- Revert one codemod false-positive: it rewrote the English word "rounded" →
  "rounded-sm" inside a releaseNotes.ts prose sentence.

Gate green: tsc(app+node) / eslint 0-err / prettier / vitest 79 / knip.
Verified in-browser: bg-accent/15 now computes a scheme-tracking color at 0.15
alpha across all 6 schemes × 2 themes; the hand-rolled glass border/shadow/
radius/backdrop-filter render unchanged under v4.

frontend/src/index.css

@@ -1,6 +1,51 @@
-@tailwind base;
-@tailwind components;
-@tailwind utilities;
+@import "tailwindcss";
+
+/* `inline` is load-bearing: the color vars (--bg/--accent/… below) are redefined at
+   runtime per html[data-scheme]/[data-theme], so utilities must inline the value and
+   resolve it at the USE site, not reference a once-computed --color-* token. This also
+   makes the v4 opacity modifier (bg-accent/40 → color-mix(… var(--accent) 40% …)) pick
+   up the active scheme's color. Same pattern as shadcn's v4 theme setup. */
+@theme inline {
+  --color-bg: var(--bg);
+  --color-surface: var(--surface);
+  --color-card: var(--card);
+  --color-border: var(--border);
+  --color-fg: var(--fg);
+  --color-muted: var(--muted);
+  --color-accent: var(--accent);
+  --color-accent-fg: var(--accent-fg);
+
+  --font-sans: Inter, system-ui, -apple-system, Segoe UI, Roboto, sans-serif;
+
+  --z-index-base: 10;
+  --z-index-menu: 20;
+  --z-index-chrome: 30;
+  --z-index-paneltab: 34;
+  --z-index-panel: 35;
+  --z-index-rail: 40;
+  --z-index-overlay: 50;
+  --z-index-popover: 60;
+  --z-index-tooltip: 100;
+  --z-index-tuner: 9999;
+}
+
+/*
+  The default border color has changed to `currentcolor` in Tailwind CSS v4,
+  so we've added these compatibility styles to make sure everything still
+  looks the same as it did with Tailwind CSS v3.
+
+  If we ever want to remove these styles, we need to add an explicit border
+  color utility to any element that depends on these defaults.
+*/
+@layer base {
+  *,
+  ::after,
+  ::before,
+  ::backdrop,
+  ::file-selector-button {
+    border-color: var(--color-gray-200, currentcolor);
+  }
+}
 
 :root {
   --font-scale: 1.06;