Upgrade build toolchain: Tailwind v4, htmx 2.0.9, Chart.js 4.5.1, Go 1.26

Tailwind v3.4.17 → v4.3.0: drop tailwind.config.js, move @source globs into
static/css/input.css (CSS-first config), rename bare `rounded` → `rounded-sm`
in templates for the renamed v4 radius scale.

Go 1.25 → 1.26.3 (toolchain directive), modernc.org/sqlite v1.50.0 → v1.50.1,
plus indirect bumps via go get -u.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

static/css/input.css

@@ -1,6 +1,10 @@
-/* Tailwind entry point. Compiled by the standalone CLI into tailwind.css:
+/* Tailwind v4 entry point. Compiled by the standalone CLI into tailwind.css:
    see the Makefile `css` target. The hand-written Veola component layer
-   lives in app.css and is loaded separately, so editing it needs no rebuild. */
-@tailwind base;
-@tailwind components;
-@tailwind utilities;
+   lives in app.css and is loaded separately, so editing it needs no rebuild.
+
+   v4 is CSS-first: utility classes are scanned from the @source globs below
+   (templ files aren't auto-detected), and theme tokens go in @theme blocks. */
+
+@import "tailwindcss";
+
+@source "../../templates/**/*.templ";