chore(deps): Tailwind 4 (#5021)

Renovate's tailwindcss bump (PR #150) plus the migration it needs:

- Theme moves from tailwind.config.js into app.css as `@theme inline`,
  mapping the same FabledSword tokens. tailwind.config.js is gone.
- PostCSS runs @tailwindcss/postcss; autoprefixer is dropped, since
  Tailwind 4 prefixes through Lightning CSS.
- Class renames from @tailwindcss/upgrade 4.3.3, reviewed: outline-none
  -> outline-hidden, focus-visible:outline -> outline-solid, shadow ->
  shadow-sm, shadow-sm -> shadow-xs, flex-shrink-0 -> shrink-0. Bare
  `rounded` stays: v4 keeps it at 0.25rem, as before.
- Three v3 preflight defaults kept in a base layer so nothing changes on
  screen: gray-200 default border colour, gray-400 placeholder text and
  the pointer cursor on buttons.
- The unused class-based dark variant is not carried over; no template
  uses `dark:`.

Clears the five high and two moderate npm audit findings that came in
through Tailwind 3 (braces, chokidar, micromatch, fast-glob,
postcss-selector-parser).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/app.css

@@ -1,8 +1,75 @@
-@import './lib/styles/tokens.generated.css';
+@import './lib/styles/tokens.generated.css' layer(base);
 
-@tailwind base;
-@tailwind components;
-@tailwind utilities;
+@import 'tailwindcss';
+
+/* The FabledSword tokens as Tailwind's theme. `inline` makes each utility emit
+ * the var(--fs-*) itself, so a theme swap that redefines the tokens reaches the
+ * utilities without going through a second variable. */
+@theme inline {
+  --color-background: var(--fs-obsidian);
+
+  --color-surface: var(--fs-iron);
+  --color-surface-hover: var(--fs-slate);
+
+  --color-border: var(--fs-pewter);
+
+  --color-text-primary: var(--fs-parchment);
+  --color-text-secondary: var(--fs-vellum);
+  --color-text-muted: var(--fs-ash);
+
+  --color-action-primary: var(--fs-moss);
+  --color-action-secondary: var(--fs-bronze);
+  --color-action-destructive: var(--fs-oxblood);
+  --color-action-fg: var(--fs-on-action);
+
+  --color-accent: var(--fs-accent);
+  --color-accent-tint: color-mix(in srgb, var(--fs-accent) 12%, transparent);
+  --color-accent-fg: var(--fs-accent-fg);
+
+  --color-warning: var(--fs-warning);
+  --color-warning-fg: var(--fs-warning-fg);
+
+  --color-error: var(--fs-error);
+  --color-error-fg: var(--fs-error-fg);
+
+  --color-info: var(--fs-info);
+  --color-info-fg: var(--fs-info-fg);
+
+  --radius-sm: var(--fs-radius-sm);
+  --radius-md: var(--fs-radius-md);
+  --radius-lg: var(--fs-radius-lg);
+  --radius-xl: var(--fs-radius-xl);
+
+  --font-display: var(--fs-font-display);
+  --font-sans: var(--fs-font-body);
+  --font-mono: var(--fs-font-mono);
+}
+
+/* Tailwind 3's preflight defaults that 4 changed (#5021). Kept so the upgrade
+ * changes nothing on screen; each can go once the elements relying on it name
+ * the value themselves.
+ * - A border with no colour utility was gray-200; 4 makes it currentcolor.
+ * - Placeholder text was gray-400; 4 makes it the text colour at 50%.
+ * - Buttons had a pointer cursor; 4 leaves the browser default. */
+@layer base {
+  *,
+  ::after,
+  ::before,
+  ::backdrop,
+  ::file-selector-button {
+    border-color: var(--color-gray-200, currentcolor);
+  }
+
+  input::placeholder,
+  textarea::placeholder {
+    color: var(--color-gray-400);
+  }
+
+  button:not(:disabled),
+  [role='button']:not(:disabled) {
+    cursor: pointer;
+  }
+}
 
 :root {
   color-scheme: dark;