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/routes/+page.svelte

@@ -143,9 +143,7 @@
 			>
 				<!-- Tag Filter -->
 				<div class="md:col-span-12">
-					<label class="block text-xs font-bold uppercase tracking-widest text-gray-500 mb-2"
-						>Schlagworte</label
-					>
+					<p class="block text-xs font-bold uppercase tracking-widest text-gray-500 mb-2">Schlagworte</p>
 					<TagInput bind:tags={tagNames} allowCreation={false} on:change={triggerSearch} />
 				</div>