fix(desktop): show private/forum icons in channel browser rows

[santhiprakash]

Fixes #6120.

Summary

The Add Channel dialog (ChannelBrowserDialog) rendered every row with a hardcoded # prefix, ignoring channel.visibility and channel.channelType. The sidebar already shows a lock for private channels and a file icon for forum channels; the channel browser drifted away from that and the same channel looked different in two surfaces of the same window.

Extract the icon-switching logic into a shared ChannelRowIcon component with a pure getChannelRowIconKind() helper for unit testing, and reuse it from both ChannelBrowserDialog and SidebarSection so the two views can't drift again.

Visibility precedence stays: private always wins, then forum, then the default # — the same precedence SidebarChannelIcon had.

Evidence

Checked against main @ 978e585e8 (chore(release): release Buzz Desktop version 0.5.16 (#6191)):

• ChannelBrowserDialog.tsx:792-798 was emitting a literal # regardless of channel kind.
• SidebarSection.tsx:242-244 already showed a Lock for visibility === private and a FileText for channelType === forum.

Reproduce

1. Create or join a private channel and a public channel in the same community.
2. Open the channel browser (Add channel).
3. Both rows show #. Nothing marks the private one as private.
4. The sidebar shows a lock on the private channel.

Expected

The browser row uses the same icon logic as the sidebar: lock for visibility === private, FileText for forum channels, # otherwise. Extracting the existing sidebar helper keeps the two views from drifting again.

Testing

cd desktop && node --import ./test-loader.mjs --experimental-strip-types \
  --test src/features/channels/ui/ChannelRowIcon.test.mjs

6/6 pass. The helper is tested directly (getChannelRowIconKind) and the component is rendered under JSDOM + @testing-library/react to assert the right lucide-* class reaches the DOM for each channel kind.

tsc --noEmit -p tsconfig.json is clean. The existing SidebarSection.test.mjs still passes (no behavior change for that surface — the helper now lives in ChannelRowIcon.tsx but is imported from the same place).

Notes

• No screenshot: the icons are already present in the sidebar and the change makes the browser match it — the sidebar is the reference.
• Pure refactor of SidebarChannelIcon's icon switch into the shared helper; no behavior change for the sidebar surface.
• File-by-file:
  • +desktop/src/features/channels/ui/ChannelRowIcon.tsx (new): 36 lines.
  • +desktop/src/features/channels/ui/ChannelRowIcon.test.mjs (new): 110 lines.
  • ~desktop/src/features/channels/ui/ChannelBrowserDialog.tsx: replace hardcoded # span with ChannelRowIcon.
  • ~desktop/src/features/sidebar/ui/SidebarSection.tsx: replace inline SidebarChannelIcon switch with ChannelRowIcon, drop three unused lucide-react imports.