Settle on one visual language

The interface had drifted into two of everything. The calendar and its
popovers were built around a blue accent the rest of the app had long since
replaced with green, five different sizes served the same class of small
label, and page sections were a stack of identical bordered cards that gave
no sense of what mattered most. Sections now separate by space and a hairline,
the overview leads with its headline figure at a size that says so, and the
palette, type scale and spacing live in one place as variables rather than
scattered literals.

Two things were not styled at all. The sign-in screen was an unstyled form in
the top-left corner, and the tip cards had no rules whatsoever despite their
markup already carrying a severity, so every tip rendered identically with a
plain green square for its dismiss control.

The calendar was also unusable on a narrow screen: the preset rail never
collapsed and covered the last three columns of the grid.

frontend/src/styles/responsive.css

@@ -155,6 +155,18 @@
   .calendar-day__number {
     font-size: var(--fs-sm);
   }
+
+  /* Pre-existing bug this pass also fixes: `.calendar__layout` never
+   * stacked, so the fixed-width preset rail sat on top of Friday/Saturday/
+   * Sunday's columns instead of beside them once the month grid had no room
+   * left. The rail moves below the grid and goes full-width. */
+  .calendar__layout {
+    flex-direction: column;
+  }
+
+  .preset-rail {
+    width: 100%;
+  }
 }
 
 /* Tablet - Medium screens (641px to 1024px) */