fix: resolve all 47 svelte-check errors and 10 a11y warnings

Root cause 1 — OpenAPI types: add @Schema(requiredMode=REQUIRED) to
non-nullable fields on Person, Tag, Document, AppUser, UserGroup;
regenerate api.ts so required fields are no longer optional.

Root cause 2 — Stale types: api.ts regenerated, picking up the Tag
endpoint fix from commit 62189d8 (List<Tag> instead of List<String>).

Root cause 3 — openapi-fetch error pattern: replace `if (apiError)`
(broken when error type is never/undefined) with `if (!result.response.ok)`
across all +page.server.ts files. Cast error via `unknown` to satisfy TS.

Root cause 4 — FormData casts: add `as string` / `as string[]` to
FormData.get() / FormData.getAll() calls in admin/+page.server.ts.

Standalone fixes:
- +page.server.ts: return error field so home page template compiles
- documents/[id]/+page.svelte: type loadFile param, remove invalid iframe `type`
- conversations: type documents as Document[] instead of unknown[]
- persons/[id]: non-null assert person data after ok-check

a11y: aria-label on all icon-only buttons in TagInput and admin page,
replace invalid <label> with <p> for compound controls, remove autofocus.

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

frontend/src/lib/components/TagInput.svelte

@@ -80,6 +80,7 @@
 				<button
 					type="button"
 					on:click={() => removeTag(i)}
+					aria-label="Schlagwort entfernen"
 					class="text-brand-navy/50 hover:text-red-500 focus:outline-none"
 				>
 					<svg class="w-3 h-3" fill="none" stroke="currentColor" viewBox="0 0 24 24"
@@ -117,12 +118,15 @@
 					class="absolute left-0 top-full mt-1 w-full bg-white border border-gray-200 rounded shadow-lg z-50 max-h-48 overflow-y-auto"
 				>
 					{#each suggestions as suggestion, i}
-						<!-- svelte-ignore a11y-click-events-have-key-events -->
 						<li
+							role="option"
+							aria-selected={i === activeIndex}
+							tabindex="0"
 							class="px-3 py-2 text-sm cursor-pointer hover:bg-brand-sand/20 {i === activeIndex
 								? 'bg-brand-sand/20 text-brand-navy font-bold'
 								: 'text-gray-700'}"
 							on:click={() => addTag(suggestion)}
+							on:keydown={(e) => e.key === 'Enter' && addTag(suggestion)}
 						>
 							{suggestion}
 						</li>