fix(tags): add accessible name to tag modal

[ChristophWurst]

NcModal requires a name or labelId for accessibility; without it Vue warns and screen readers get no dialog label;

[Vue warn]: [NcModal] You need either set the name or set a `labelId` for accessibility.

found in

---> <NcModal>
       <TagModal> at src/components/TagModal.vue
         <RouterLink>
           <EnvelopeSkeleton> at src/components/EnvelopeSkeleton.vue
             <Envelope> at src/components/Envelope.vue
               <TransitionGroup>
                 <EnvelopeList> at src/components/EnvelopeList.vue
                   <Mailbox> at src/components/Mailbox.vue
                     <NcAppContentList>
                       <Pane>
                         <Splitpanes>
                           <NcAppContent>
                             <MailboxThread> at src/components/MailboxThread.vue
                               <NcContent>
                                 <Home> at src/views/Home.vue
                                   <App> at src/App.vue
                                     <Root>

Assisted-by: Claude:claude-opus-4-8

Discovered while testing #13257.

How to test

1. Open the three dot menu of a thread envelope
2. Click Edit tags

main: console Vue warning
here: ✨

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[ChristophWurst]

/backport to stable5.10

[odzhychko]

LGTM, if the visual side effect was intended/acceptable.

Has side effect of making the backdrop darker.
Left before, right after:

Accessibility works as expected.

[ChristophWurst]

Has side effect of making the backdrop darker.
Left before, right after:

That seems to be intended: https://github.com/nextcloud-libraries/nextcloud-vue/blob/5de5b9ce20c8b85d929837498d2cfe9f05c76747/src/components/NcModal/NcModal.vue#L640-L649

[ChristophWurst]

Pushed an alternative that now reference an existing heading as modal accessibility label. This way we can keep the light opacity of the backdrop.

[odzhychko]

New changes look also good.
No visual difference to main.
But added accessibility.