refactor(sdd-webapp): extract constants and unify type declarations

- Add REMOVE_PROJECT_VALUE constant in ProjectSelector
- Add DEFAULT_ERROR_MESSAGE constant in useSpecs
- Change interface to type for ProjectSelectorProps
- Use getErrorMessage in ProjectSelector handleAdd

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: masseater

packages/sdd-webapp/src/client/components/ProjectSelector.tsx

@@ -7,14 +7,17 @@ import {
 	PopoverTrigger,
 } from "@/components/ui/popover";
 import type { Project } from "../../types";
+import { getErrorMessage } from "../lib/error";
 
-interface ProjectSelectorProps {
+const REMOVE_PROJECT_VALUE = "__remove__" as const;
+
+type ProjectSelectorProps = {
 	projects: Project[];
 	selectedProject: Project | null;
 	onSelect: (project: Project | null) => void;
 	onAdd: (path: string, name?: string) => Promise<Project>;
 	onRemove: (id: string) => Promise<void>;
-}
+};
 
 export function ProjectSelector({
 	projects,
@@ -31,7 +34,7 @@ export function ProjectSelector({
 
 	const handleChange = (e: ChangeEvent<HTMLSelectElement>) => {
 		const value = e.target.value;
-		if (value === "__remove__") {
+		if (value === REMOVE_PROJECT_VALUE) {
 			if (
 				selectedProject &&
 				confirm(`Remove "${selectedProject.name}" from the list?`)
@@ -57,7 +60,7 @@ export function ProjectSelector({
 			setError(null);
 			setShowAddForm(false);
 		} catch (err) {
-			setError(err instanceof Error ? err.message : "Failed to add project");
+			setError(getErrorMessage(err, "Failed to add project"));
 		} finally {
 			setSubmitting(false);
 		}
@@ -77,7 +80,7 @@ export function ProjectSelector({
 					</option>
 				))}
 				{selectedProject && (
-					<option value="__remove__">- Remove Current Project</option>
+					<option value={REMOVE_PROJECT_VALUE}>- Remove Current Project</option>
 				)}
 			</select>
 			<Popover

packages/sdd-webapp/src/client/hooks/useSpecs.ts

@@ -4,6 +4,8 @@ import { fetchJson } from "../lib/api";
 import { getErrorMessage } from "../lib/error";
 import { apiRoutes } from "../lib/routes";
 
+const DEFAULT_ERROR_MESSAGE = "Unknown error";
+
 export function useSpecs(
 	projectId: string | null,
 	options?: { includeArchived?: boolean },
@@ -34,7 +36,7 @@ export function useSpecs(
 			);
 			setFileTree(data);
 		} catch (err) {
-			setError(getErrorMessage(err, "Unknown error"));
+			setError(getErrorMessage(err, DEFAULT_ERROR_MESSAGE));
 		} finally {
 			stopLoading();
 		}
@@ -52,7 +54,7 @@ export function useSpecs(
 				setSpecContent(data);
 				setSelectedFile(path);
 			} catch (err) {
-				setError(getErrorMessage(err, "Unknown error"));
+				setError(getErrorMessage(err, DEFAULT_ERROR_MESSAGE));
 			} finally {
 				stopLoading();
 			}
@@ -72,7 +74,7 @@ export function useSpecs(
 			);
 			setStatus(data);
 		} catch (err) {
-			setError(getErrorMessage(err, "Unknown error"));
+			setError(getErrorMessage(err, DEFAULT_ERROR_MESSAGE));
 		} finally {
 			stopLoading();
 		}