UI: light Things-3 design system — tokens, task rows, chips, home

Replaces the dark navy palette with a light grouped-background system:
- theme.ts: full token set (space/radius/type/shadow) + light+dark palettes
- ui.tsx: Screen/Title/SectionHeader/Card/Separator/Pill/Empty primitives
- TaskRow: rounded-square checkbox carrying priority colour, inset separators
- TaskListScreen: rows grouped into a card, drag lift, refined deferred footer
- Icon.tsx: dependency-free geometric icon set (emoji don't take a tint)
- home: large title, Next-up card, "needs attention" rows, icon tab bar
- Priority/Estimate/Date chips unified on the accent-soft active state

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/src/lib/priority.ts

@@ -9,11 +9,21 @@ export const PRIORITY_LABEL: Record<Task["priority"], string> = {
   4: "Low",
 };
 
+// Todoist's flag mapping (urgent red → low grey), retuned to sit calmly on a
+// light canvas. `PRIORITY_TINT` is the same hue at wash strength, for the
+// selected state of a chip — a saturated fill would shout on a white card.
 export const PRIORITY_COLOR: Record<Task["priority"], `#${string}`> = {
-  1: "#E05B5B",
-  2: "#E0A93E",
-  3: "#5B8DEF",
-  4: "#6B7280",
+  1: "#E5484D",
+  2: "#D98324",
+  3: "#2F6FED",
+  4: "#9A9AA2",
+};
+
+export const PRIORITY_TINT: Record<Task["priority"], `#${string}`> = {
+  1: "#FDECEC",
+  2: "#FBF0E2",
+  3: "#E8F0FE",
+  4: "#F0F0F3",
 };
 
 export function formatEstimate(min: number | null): string | null {