feat: reply mention indicator

[mini-bomba]

Description

in yet another episode of "mini_bomba yoinks discord features into sable":
reply mention indicators (the @ before the username, not the yellow highlight)

proudly vibecoded using my brain, while referencing matrix client docs

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[nushea]

huh didnt notice that was a thing on discord maybe it could be made into a (cut-out) bell icon tho? like if its muted if its not too much writing, like i think that might be confusing

[dozro]

code lgtm