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>

BACKLOG.md

@@ -15,15 +15,6 @@ In dark mode the "Clear" button text lightens on hover, giving visual feedback.
 mode and Dan mode it currently does nothing visible. Should have a matching subtle
 highlight (e.g. text darkens, underline appears, or border darkens).
 
-### Admin: allow renaming unit and group IDs
-The edit forms do not currently expose the `id` field for existing units or groups.
-
-- **Units:** straightforward — update `units[].id` and save.
-- **Groups:** update `groups[].id` and rewrite every `units[].group` reference in the
-  same atomic `saveData()` call.
-
-Validate that the new ID is unique and kebab-safe before committing in both cases.
-
 ### Admin: tooltips / descriptions for units
 Allow users to attach an optional `description` string to each unit. On the front end,
 show it as a tooltip when hovering over the unit name or symbol. A single `description`