feat(09-01): create TextFieldMD3 component with floating label + MD3 button constants

- Add TextFieldMD3.tsx: outlined text field with CSS-only floating label via peer/:not(:placeholder-shown)
- Add TextFieldMD3.test.tsx: 9 tests covering htmlFor/id pairing, registration spread, error/helpText/suffix/required slots
- Add src/styles/md3-buttons.ts: MD3_BTN_FILLED, MD3_BTN_OUTLINED, MD3_BTN_TEXT class constants

src/styles/md3-buttons.ts

@@ -0,0 +1,26 @@
+/**
+ * MD3 button class constants.
+ * Apply these className strings to <button> elements directly.
+ *
+ * - MD3_BTN_FILLED:   primary action (Next, Download All)
+ * - MD3_BTN_OUTLINED: secondary action (Back, Copy, Download per file)
+ * - MD3_BTN_TEXT:     tertiary action
+ */
+
+export const MD3_BTN_FILLED =
+  'px-6 py-2.5 rounded-full bg-primary text-on-primary text-sm font-medium ' +
+  'hover:opacity-90 active:opacity-80 transition-opacity ' +
+  'disabled:opacity-40 disabled:cursor-not-allowed ' +
+  'focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-primary/50';
+
+export const MD3_BTN_OUTLINED =
+  'px-6 py-2.5 rounded-full border border-outline text-on-surface text-sm font-medium ' +
+  'hover:bg-primary/8 active:bg-primary/12 transition-colors ' +
+  'disabled:opacity-40 disabled:cursor-not-allowed ' +
+  'focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-primary/50';
+
+export const MD3_BTN_TEXT =
+  'px-4 py-2.5 rounded-full text-primary text-sm font-medium ' +
+  'hover:bg-primary/8 active:bg-primary/12 transition-colors ' +
+  'disabled:opacity-40 disabled:cursor-not-allowed ' +
+  'focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-primary/50';