Implement native MSC2545 image packs, space-level pack management, and instant preloading

[guillemo12]

Checklist

• I have read through review guidelines and
  CONTRIBUTING.md.

• Tests written for new code (and old code if feasible).

• New or updated public/exported symbols have accurate TSDoc documentation.

• Linter and other CI checks pass.

• I have licensed the changes to Element by completing the [Contributor License Agreement (CLA)](https://cla-assistant.
  io/element-hq/element-web)

  ---

Summary

Replaces the legacy widget-based sticker picker with a native Matrix implementation of MSC2545 (Image Packs for Emoticons
and Stickers), adding space-level sticker pack management, hierarchical inheritance across nested spaces, background
preloading for instant sticker display, and reliable pack deletion.

What & Why

• Native MSC2545 Image Packs (ImagePackStore.ts, ImagePacksPicker.tsx): Full implementation of the MSC2545
  specification for emoticons and stickers. Emoticons can be inserted as shortcodes into the composer, and stickers are sent as
  native m.sticker events.

• Space-Level Sticker Management (SpaceSettingsStickersTab.tsx): Adds a dedicated "Stickers & Emotes" tab to Space
  Settings, allowing space administrators to create, edit, upload, and delete sticker/emoticon packs directly in spaces.

• Hierarchical Space Inheritance: As recommended in MSC2545, child rooms and sub-spaces recursively inherit sticker
  packs from parent spaces and ancestors in the space tree.

• Background Preloading: Sticker images and avatar thumbnails are prefetched asynchronously into the browser cache on
  room change and on button hover, completely eliminating image pop-in/lag when opening the picker.

• Reliable Pack Deletion & Room Targeting: Deletion targets the pack's originating space/room rather than the active
  chat room, preventing accidental sibling deletions. Replaced native dialogs with standard QuestionDialog modals.

• Backwards Compatibility: Listens and dual-publishes to both standard m.room.image_pack and legacy im.ponies. room_emotes.

  Notes: Implement native MSC2545 image packs support for stickers and emoticons, space-level management, and instant
  background preloading.

Testing Strategy

1. Open a Space settings dialog -> Navigate to the new "Stickers & Emotes" tab.
2. Create a new sticker pack and upload images into it.
3. Open any child room or nested sub-space belonging to that Space.
4. Click the stickers/emotes button in the message composer: verify that all stickers appear instantly with preloaded
   images.
5. Click a sticker to send it as m.sticker into the chat timeline.
6. Open "Manage Image Packs" and delete a pack, confirming that the QuestionDialog modal appears and cleans up state
   properly.

[CLAassistant]

All committers have signed the CLA.