Make button labels visible on non-green buttons

The global `button` rule pairs a green background with white text. Any
variant that overrode the background without also setting `color` kept
the white text and rendered invisible against a light background.

Four controls were affected, all measured white-on-white or
white-on-light-grey: the calendar header's prev/next arrows, the
calendar's override-mode toggle, the red-days list's year navigation,
and the modal close button.

Each now declares its own `color: var(--c-text)`. Using the variable
rather than a literal means the high-contrast media query in
responsive.css, which redefines --c-text to #000, still applies.

Also adds the hover and focus-visible states the Settings and
vacation-days Save buttons were missing entirely -- they were the only
green buttons in the app that did not react to the pointer. Hover is
scoped with :not(:disabled) so a button disabled mid-save stays inert.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RXMhc9nT6sNz1SL4j8dznL

frontend/src/pages/calendar.css

@@ -16,6 +16,7 @@
   min-height: 36px;
   padding: 8px 12px;
   background: none;
+  color: var(--c-text);
   border: 1px solid #d0d0d0;
   border-radius: 4px;
   font-size: 18px;
@@ -373,6 +374,7 @@
   border: 1px solid #d0d0d0;
   border-radius: 4px;
   background: #fff;
+  color: var(--c-text);
   cursor: pointer;
   font-size: 13px;
   transition: background-color 0.2s, border-color 0.2s, color 0.2s;