Switch from telegram_bot_username to telegram_notifications in /config endpoint

[paskal]

Bot username is returned as an answer to subscribe request, so knowing it in advance is unnecessary. Needed for #830.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +4.25 🎉

Comparison is base (d3fdd7b) 58.63% compared to head (f3927fc) 62.88%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1648      +/-   ##
==========================================
+ Coverage   58.63%   62.88%   +4.25%     
==========================================
  Files         128      136       +8     
  Lines        2877     3387     +510     
  Branches      737      804      +67     
==========================================
+ Hits         1687     2130     +443     
  Misses       1065     1065              
- Partials      125      192      +67     

Impacted Files	Coverage Δ
frontend/apps/remark42/app/common/static-store.ts	100.00% <ø> (ø)
frontend/apps/remark42/app/common/types.ts	100.00% <ø> (ø)
frontend/packages/api/clients/public.ts	92.81% <100.00%> (ø)

... and 7 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[github-actions]

size-limit report 📦

Path	Size
public/embed.mjs	2.04 KB (0%)
public/remark.mjs	73.34 KB (-0.02% 🔽)
public/remark.css	8.12 KB (0%)
public/last-comments.mjs	35.92 KB (-0.03% 🔽)
public/last-comments.css	3.72 KB (+0.03% 🔺)
public/deleteme.mjs	12.12 KB (-0.07% 🔽)
public/counter.mjs	749 B (0%)