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.server.ts

@@ -44,7 +44,8 @@ export async function load({ url, fetch }) {
                 senderName: senderObj ? `${senderObj.firstName} ${senderObj.lastName}` : '',
                 receiverName: receiverObj ? `${receiverObj.firstName} ${receiverObj.lastName}` : ''
             },
-            filters: { q, from, to, senderId, receiverId, tags }
+            filters: { q, from, to, senderId, receiverId, tags },
+            error: null as string | null
         };
     } catch (e) {
         if ((e as { status?: number }).status) throw e;
@@ -52,7 +53,8 @@ export async function load({ url, fetch }) {
         return {
             documents: [],
             initialValues: { senderName: '', receiverName: '' },
-            filters: { q, from, to, senderId, receiverId, tags }
+            filters: { q, from, to, senderId, receiverId, tags },
+            error: 'Daten konnten nicht geladen werden.' as string | null
         };
     }
 }