feat: redesign detail page with stage modal and milestone APIs

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/prisma/migrations/20260605120000_milestone_stage_fields/migration.sql

@@ -0,0 +1,15 @@
+-- AlterTable
+ALTER TABLE "milestones" ADD COLUMN IF NOT EXISTS "startDate" TIMESTAMP(3);
+ALTER TABLE "milestones" ADD COLUMN IF NOT EXISTS "links" TEXT;
+
+-- AlterTable
+ALTER TABLE "files" ADD COLUMN IF NOT EXISTS "milestoneId" TEXT;
+CREATE INDEX IF NOT EXISTS "files_milestoneId_idx" ON "files"("milestoneId");
+ALTER TABLE "files" ADD CONSTRAINT "files_milestoneId_fkey"
+  FOREIGN KEY ("milestoneId") REFERENCES "milestones"("id") ON DELETE SET NULL ON UPDATE CASCADE;
+
+-- AlterTable
+ALTER TABLE "task_details" ADD COLUMN IF NOT EXISTS "milestoneId" TEXT;
+CREATE INDEX IF NOT EXISTS "task_details_milestoneId_idx" ON "task_details"("milestoneId");
+ALTER TABLE "task_details" ADD CONSTRAINT "task_details_milestoneId_fkey"
+  FOREIGN KEY ("milestoneId") REFERENCES "milestones"("id") ON DELETE CASCADE ON UPDATE CASCADE;