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/generated/api.ts

@@ -282,14 +282,14 @@ export interface components {
     schemas: {
         Tag: {
             /** Format: uuid */
-            id?: string;
-            name?: string;
+            id: string;
+            name: string;
         };
         Person: {
             /** Format: uuid */
-            id?: string;
-            firstName?: string;
-            lastName?: string;
+            id: string;
+            firstName: string;
+            lastName: string;
             alias?: string;
         };
         DocumentUpdateDTO: {
@@ -307,13 +307,13 @@ export interface components {
         };
         Document: {
             /** Format: uuid */
-            id?: string;
-            title?: string;
+            id: string;
+            title: string;
             filePath?: string;
             contentType?: string;
-            originalFilename?: string;
+            originalFilename: string;
             /** @enum {string} */
-            status?: "PLACEHOLDER" | "UPLOADED" | "TRANSCRIBED" | "REVIEWED" | "ARCHIVED";
+            status: "PLACEHOLDER" | "UPLOADED" | "TRANSCRIBED" | "REVIEWED" | "ARCHIVED";
             /** Format: date */
             documentDate?: string;
             location?: string;
@@ -323,9 +323,9 @@ export interface components {
             transcription?: string;
             summary?: string;
             /** Format: date-time */
-            createdAt?: string;
+            createdAt: string;
             /** Format: date-time */
-            updatedAt?: string;
+            updatedAt: string;
             receivers?: components["schemas"]["Person"][];
             sender?: components["schemas"]["Person"];
             tags?: components["schemas"]["Tag"][];
@@ -338,20 +338,20 @@ export interface components {
         };
         AppUser: {
             /** Format: uuid */
-            id?: string;
-            username?: string;
+            id: string;
+            username: string;
             password?: string;
             email?: string;
-            enabled?: boolean;
-            groups?: components["schemas"]["UserGroup"][];
+            enabled: boolean;
+            groups: components["schemas"]["UserGroup"][];
             /** Format: date-time */
-            createdAt?: string;
+            createdAt: string;
         };
         UserGroup: {
             /** Format: uuid */
-            id?: string;
-            name?: string;
-            permissions?: string[];
+            id: string;
+            name: string;
+            permissions: string[];
         };
         GroupDTO: {
             name?: string;
@@ -738,7 +738,7 @@ export interface operations {
                     [name: string]: unknown;
                 };
                 content: {
-                    "*/*": string[];
+                    "*/*": components["schemas"]["Tag"][];
                 };
             };
         };