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/documents/[id]/edit/+page.server.ts

@@ -12,16 +12,16 @@ export async function load({ params, fetch }) {
         api.GET('/api/persons')
     ]);
 
-    if (docResult.error) {
-        const code = (docResult.error as { code?: string })?.code;
+    if (!docResult.response.ok) {
+        const code = (docResult.error as unknown as { code?: string })?.code;
         throw error(docResult.response.status, getErrorMessage(code));
     }
-    if (personsResult.error) {
+    if (!personsResult.response.ok) {
         throw error(personsResult.response.status, getErrorMessage('INTERNAL_ERROR'));
     }
 
     return {
-        document: docResult.data,
+        document: docResult.data!,
         persons: personsResult.data
     };
 }