fix(errors): ValidationError for domain failures, runGuarded boundary

- AdvanceCycleUseCase and ReorderSubjectsUseCase now throw ValidationError
  instead of plain Error for domain rule violations.
- Add DomHelpers.runGuarded error-boundary helper and use it in
  registerCommands (advance cycle, cycle snapshot, summary) to remove
  repeated try/catch + Notice boilerplate.

Closes #6

src/application/use-cases/AdvanceCycleUseCase.ts

@@ -4,7 +4,7 @@ import { ActiveCycleSnapshot } from "@/application/use-cases/GetActiveCycleSnaps
 import { CycleService } from "@/domain/services/CycleService";
 import { ItemProgressService } from "@/domain/services/ItemProgressService";
 import { ActiveContestGuard } from "@/application/guards/ActiveContestGuard";
-import { NotFoundError } from "@/domain/errors/DomainErrors";
+import { NotFoundError, ValidationError } from "@/domain/errors/DomainErrors";
 
 /**
  * Use case for advancing the study cycle to the next subject.
@@ -38,7 +38,7 @@ export class AdvanceCycleUseCase {
     );
 
     if (!nextSubject) {
-      throw new Error(`Contest "${activeContestId}" has no active subjects.`);
+      throw new ValidationError(`Contest "${activeContestId}" has no active subjects.`);
     }
 
     const subjectItems = data.studyItems.filter(

src/application/use-cases/ReorderSubjectsUseCase.ts

@@ -37,14 +37,14 @@ export class ReorderSubjectsUseCase {
       .filter((subject) => subject.contestId === input.contestId);
 
     if (contestSubjects.length !== input.subjectIdsInOrder.length) {
-      throw new Error("The provided subject order does not match the contest subject list.");
+      throw new ValidationError("The provided subject order does not match the contest subject list.");
     }
 
     const subjectIdSet = new Set(contestSubjects.map((subject) => subject.id));
 
     for (const subjectId of input.subjectIdsInOrder) {
       if (!subjectIdSet.has(subjectId)) {
-        throw new Error(`Subject "${subjectId}" does not belong to contest "${input.contestId}".`);
+        throw new ValidationError(`Subject "${subjectId}" does not belong to contest "${input.contestId}".`);
       }
     }
 

src/ui/commands/registerCommands.ts

@@ -2,6 +2,7 @@ import type { Plugin } from "obsidian";
 import { Notice } from "obsidian";
 
 import type { PluginDataStore } from "@/application/ports/PluginDataStore";
+import { DomHelpers } from "@/ui/view/shared/DomHelpers";
 import { AdvanceCycleUseCase } from "@/application/use-cases/AdvanceCycleUseCase";
 import { CreateContestUseCase } from "@/application/use-cases/CreateContestUseCase";
 import { CreateStudyItemUseCase } from "@/application/use-cases/CreateStudyItemUseCase";
@@ -184,21 +185,18 @@ export function registerCommands(plugin: Plugin, dataStore: PluginDataStore): vo
   plugin.addCommand({
     id: "leif-advance-cycle",
     name: "Advance cycle",
-    callback: async () => {
-      try {
+    callback: () =>
+      DomHelpers.runGuarded(async () => {
         const state = await advanceCycle.execute();
         new Notice(`Current subject: ${state.currentSubjectId ?? "none"}`);
-      } catch (error) {
-        new Notice(error instanceof Error ? error.message : "Could not advance cycle.");
-      }
-    }
+      }, "Could not advance cycle.")
   });
 
   plugin.addCommand({
     id: "leif-show-cycle-snapshot",
     name: "Show cycle snapshot",
-    callback: async () => {
-      try {
+    callback: () =>
+      DomHelpers.runGuarded(async () => {
         const snapshot = await getActiveCycleSnapshot.execute();
         const data = await dataStore.load();
         const itemMap = new Map(data.studyItems.map((item) => [item.id, item.title]));
@@ -210,10 +208,7 @@ export function registerCommands(plugin: Plugin, dataStore: PluginDataStore): vo
         new Notice(
           `Current: ${currentLabel} | Next: ${nextLabel} | Current item: ${currentItemLabel} | Next item: ${nextItemLabel}`
         );
-      } catch (error) {
-        new Notice(error instanceof Error ? error.message : "Could not read cycle snapshot.");
-      }
-    }
+      }, "Could not read cycle snapshot.")
   });
 
   plugin.addCommand({
@@ -237,8 +232,8 @@ export function registerCommands(plugin: Plugin, dataStore: PluginDataStore): vo
   plugin.addCommand({
     id: "leif-show-summary",
     name: "Show active contest summary",
-    callback: async () => {
-      try {
+    callback: () =>
+      DomHelpers.runGuarded(async () => {
         const summary = await getActiveContestSummary.execute();
 
         if (summary.subjectSummaries.length === 0) {
@@ -258,10 +253,7 @@ export function registerCommands(plugin: Plugin, dataStore: PluginDataStore): vo
           .join(" | ");
 
         new Notice(message);
-      } catch (error) {
-        new Notice(error instanceof Error ? error.message : "Could not load active contest summary.");
-      }
-    }
+      }, "Could not load active contest summary.")
   });
 
   plugin.addCommand({

src/ui/view/shared/DomHelpers.ts

@@ -493,6 +493,23 @@ export class DomHelpers {
     new Notice(error instanceof Error ? error.message : fallbackMessage);
   }
 
+  /**
+   * Error-boundary helper: runs an async action and surfaces any failure
+   * through notifyError so callers don't repeat try/catch + Notice boilerplate.
+   */
+  static runGuarded(
+    action: () => void | Promise<void>,
+    fallbackMessage: string
+  ): Promise<void> {
+    return (async () => {
+      try {
+        await action();
+      } catch (error) {
+        this.notifyError(error, fallbackMessage);
+      }
+    })();
+  }
+
   /**
    * Creates a modal overlay with a centered card.
    * Implements role=dialog, aria-modal, a focus trap and Escape-to-close.