feat: two-line cards, responsive grid, real uniform font size

- ConversionResult: already two-line layout (value + label/symbol)
- app.css: responsive grid (1/2/3 cols at <480/480-899/≥900px)
- uniformFontSize.ts: JS-measured uniform font size per grid
- config.ts: remove MIN_COLUMN_WIDTH, grid is now CSS-only
- BACKLOG.md: remove resolved 'allow renaming unit/group IDs' item

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/config.ts

@@ -11,12 +11,12 @@ export const FRACTIONAL_DIGITS: number = 8;
 
 /**
  * Output grid layout configuration.
- * Grid uses CSS Grid repeat(auto-fill, minmax(MIN_COLUMN_WIDTH, 1fr)).
- * Column count is automatic — determined by available width ÷ (MIN_COLUMN_WIDTH + GRID_GAP).
+ * Grid uses CSS media-query breakpoints for responsive column counts:
+ *   <480px  → 1 column
+ *   480–899px → 2 columns
+ *   ≥900px  → 3 columns
+ * See app.css .results-grid for the actual CSS.
  */
 
-/** Minimum tile width for the output grid. Controls automatic column count. */
-export const MIN_COLUMN_WIDTH: string = '160px';
-
 /** Gap between output grid tiles. Used for both row and column gaps. */
 export const GRID_GAP: string = '1rem';