fix(avatars): draw guest and bot avatars on the client

[AndyScherzinger]

Guests, email participants and bots have no avatar on the server, yet the app requested one from /index.php/avatar/guest/{name} in six places and drew its own initial or ">" in others, each place with its own idea of when a display name counts as a real name.

Resolve all of them in CharacterAvatarUtils, mirroring the web client's AvatarWrapper so both clients show the same avatar for the same actor: a named guest gets the first character of their name, a bot the shell prompt, an unnamed guest the generic person icon, and the changelog and sample bots keep the app icon. Two renderers consume that decision, CharacterAvatarDrawable for views and notifications and CharacterAvatar for Compose, both scaling the character to the bounds they are given instead of to a fixed bitmap, and both taking the theme-aware colours web uses.

Removes ApiUtils.getUrlForGuestAvatar together with the requests for it, TextDrawable, and the already unused ChatMessageUtils.

Assisted-by: Claude Code:claude-opus-5

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔖 Capability is checked or not needed
• 🔙 Backport requests are created or not needed: /backport to stable-xx.x
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

🤖 AI (if applicable)

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

[github-actions]

APK file: https://github.com/nextcloud/talk-android/actions/runs/33540018116/artifacts/9815269872
To test this change/fix you can simply download above APK file and install and test it in parallel to your existing Nextcloud app.
(please click on link to get QR code displayed)

[mahibi]

/backport to stable-25.0.x

[backportbot]

The backport to stable-25.0.x failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable-25.0.x
git pull origin stable-25.0.x

# Create the new backport branch
git checkout -b backport/6613/stable-25.0.x

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick e841d044

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/6613/stable-25.0.x

Error: Failed to clone repository: Failed to create working tree: error: RPC failed; HTTP 401 curl 22 The requested URL returned error: 401
fatal: expected flush after ref listing

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[mahibi]

/backport to stable-25.0.x