fix(frontend): enforce lint locally and in CI, fix all pre-existing violations

## Pre-commit hook
- Add .husky/pre-commit at repo root: runs `cd frontend && npm run lint`
- Update prepare script in package.json to auto-configure git hooks path
  on npm install (git -C .. config core.hooksPath .husky)
- Add lint step to CI unit-tests job so it catches issues before tests run
- Add generated dirs to .prettierignore (paraglide_bak*, test-results, .auth)
- Add src/lib/paraglide_bak* to .gitignore so ESLint can ignore them

## ESLint fixes (all pre-existing)
- Disable svelte/no-navigation-without-resolve: false positive in SvelteKit
  (rule targets Svelte 5 standalone routing, not SvelteKit <a href>)
- Fix svelte/require-each-key: add (item.id)/(item) keys to all {#each} blocks
  across 10 files — improves Svelte reconciliation performance
- Fix svelte/prefer-writable-derived in PersonTypeahead: $state+$effect → $derived
- Fix svelte/prefer-svelte-reactivity: URLSearchParams → SvelteURLSearchParams,
  Map → SvelteMap (enables Svelte reactive tracking)
- Fix @typescript-eslint/no-unused-vars: remove dead imports/variables

## Prettier
- Run npm run format to bring all source files in line with .prettierrc

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/errors.ts

@@ -5,20 +5,20 @@ import * as m from '$lib/paraglide/messages.js';
  * Keep in sync with backend/src/main/java/org/raddatz/familienarchiv/exception/ErrorCode.java
  */
 export type ErrorCode =
-    | 'DOCUMENT_NOT_FOUND'
-    | 'DOCUMENT_NO_FILE'
-    | 'FILE_NOT_FOUND'
-    | 'FILE_UPLOAD_FAILED'
-    | 'USER_NOT_FOUND'
-    | 'IMPORT_ALREADY_RUNNING'
-    | 'UNAUTHORIZED'
-    | 'FORBIDDEN'
-    | 'VALIDATION_ERROR'
-    | 'INTERNAL_ERROR';
+	| 'DOCUMENT_NOT_FOUND'
+	| 'DOCUMENT_NO_FILE'
+	| 'FILE_NOT_FOUND'
+	| 'FILE_UPLOAD_FAILED'
+	| 'USER_NOT_FOUND'
+	| 'IMPORT_ALREADY_RUNNING'
+	| 'UNAUTHORIZED'
+	| 'FORBIDDEN'
+	| 'VALIDATION_ERROR'
+	| 'INTERNAL_ERROR';
 
 export interface BackendError {
-    code: ErrorCode;
-    message: string; // English developer message — not shown to users
+	code: ErrorCode;
+	message: string; // English developer message — not shown to users
 }
 
 /**
@@ -26,29 +26,39 @@ export interface BackendError {
  * Returns null if the body is not valid JSON or does not contain a code field.
  */
 export async function parseBackendError(res: Response): Promise<BackendError | null> {
-    try {
-        const body = await res.json();
-        if (body && typeof body.code === 'string') {
-            return body as BackendError;
-        }
-    } catch {
-        // Body was not JSON
-    }
-    return null;
+	try {
+		const body = await res.json();
+		if (body && typeof body.code === 'string') {
+			return body as BackendError;
+		}
+	} catch {
+		// Body was not JSON
+	}
+	return null;
 }
 
 /** Returns a localised message for the given error code via Paraglide. Falls back to INTERNAL_ERROR. */
 export function getErrorMessage(code: ErrorCode | string | undefined): string {
-    switch (code) {
-        case 'DOCUMENT_NOT_FOUND':    return m.error_document_not_found();
-        case 'DOCUMENT_NO_FILE':      return m.error_document_no_file();
-        case 'FILE_NOT_FOUND':        return m.error_file_not_found();
-        case 'FILE_UPLOAD_FAILED':    return m.error_file_upload_failed();
-        case 'USER_NOT_FOUND':        return m.error_user_not_found();
-        case 'IMPORT_ALREADY_RUNNING':return m.error_import_already_running();
-        case 'UNAUTHORIZED':          return m.error_unauthorized();
-        case 'FORBIDDEN':             return m.error_forbidden();
-        case 'VALIDATION_ERROR':      return m.error_validation_error();
-        default:                      return m.error_internal_error();
-    }
+	switch (code) {
+		case 'DOCUMENT_NOT_FOUND':
+			return m.error_document_not_found();
+		case 'DOCUMENT_NO_FILE':
+			return m.error_document_no_file();
+		case 'FILE_NOT_FOUND':
+			return m.error_file_not_found();
+		case 'FILE_UPLOAD_FAILED':
+			return m.error_file_upload_failed();
+		case 'USER_NOT_FOUND':
+			return m.error_user_not_found();
+		case 'IMPORT_ALREADY_RUNNING':
+			return m.error_import_already_running();
+		case 'UNAUTHORIZED':
+			return m.error_unauthorized();
+		case 'FORBIDDEN':
+			return m.error_forbidden();
+		case 'VALIDATION_ERROR':
+			return m.error_validation_error();
+		default:
+			return m.error_internal_error();
+	}
 }