fix(build): use zod v3 two-arg form for z.record across all shared schemas

Same fix as the agent schema commit — zod 3.25 (resolved by ^3.25.0)
requires z.record() to take both key and value schemas. The single-arg
form is v4 syntax.

Fixed 15 remaining call sites across canvas, project, report, common,
webhook, habit, task, note, and milestone schemas. Pure type-only fix —
runtime behavior identical.

packages/shared/src/schemas/canvas.ts

@@ -56,7 +56,7 @@ export const canvasSchema = z.object({
     zoom: z.number().positive().default(1),
   }).optional(),
   background: z.string().optional(),
-  custom_fields: z.record(z.unknown()).optional(),
+  custom_fields: z.record(z.string(), z.unknown()).optional(),
   created: z.string().datetime(),
   updated: z.string().datetime(),
 });