fix(bienvenida,grabaciones): headline size, CTA radius, storage caption

Screen 14 / audit 14.3+14.7: welcome headline now 34px/ls-1.2 (was
headlineMedium's 28/ls-1.0), CTA restyled to a radius-18 rounded
rectangle instead of Material 3's default StadiumBorder (t4:696,715).

Screen 12 / audit 12.2+12.3: the recordings storage card now shows the
bold "X of Y used" headline ABOVE a 6px/radius-3 bar, followed by a
real folder-path + purge-policy caption below it -- was the bar first
with the "used" string as its only (small, generic) caption (t4:613).
New ARB key recordingsLibraryStorageFolderCaption, translated to all
13 locales.

Item 12.5 (folder/max-size settings living behind the header action
instead of inline on this screen) is a deliberate structural split
from WU15b; the audit itself scopes it out of this pass.

lib/pantallas/pantalla_bienvenida.dart

@@ -92,10 +92,16 @@ class PantallaBienvenida extends StatelessWidget {
                           const SizedBox(height: 22),
                           Text(
                             l10n.welcomeHeadline,
+                            // Audit 14.3 (t4 line 696): 34px/ls-1.2, not
+                            // headlineMedium's 28/ls-1.0 — a local override,
+                            // matching the precedent already set by every
+                            // other one-off size correction in this screen
+                            // (14.4-14.6 below).
                             style: theme.textTheme.headlineMedium?.copyWith(
+                              fontSize: 34,
                               fontWeight: FontWeight.w800,
                               height: 1.05,
-                              letterSpacing: -1.0,
+                              letterSpacing: -1.2,
                             ),
                           ),
                           const SizedBox(height: 12),
@@ -143,6 +149,14 @@ class PantallaBienvenida extends StatelessWidget {
                     height: 58,
                     child: FilledButton(
                       onPressed: () => _empezar(context),
+                      // Audit 14.7 (t4 line 715): radius 18 — Material 3's
+                      // default FilledButton shape is a fully-round
+                      // StadiumBorder, which the prototype does not draw.
+                      style: FilledButton.styleFrom(
+                        shape: RoundedRectangleBorder(
+                          borderRadius: BorderRadius.circular(18),
+                        ),
+                      ),
                       child: Text(
                         l10n.welcomeCtaLabel,
                         style: const TextStyle(