fix(channels): mobile channel rows don't show the unread badge #155

Summary

On mobile widths the channel list shows no unread badge. The unread count is kept in state (e.g. unread: 3 survives a refresh with #153), but the mobile row renderer never displays it. Found while reviewing #153; pre-existing on master.

Where (master e829d1ea)

public/channels.js:

• Desktop rows: renderChannelRow() (around line 1909) renders <span class="ch-unread-badge" ...> when ch.unread > 0 (around line 1973).
• Mobile rows: renderChannelRowMobile() (around line 2007, the regression(channels mobile): #1227 picker rows too cramped — restore prod's roomier layout (keep header wins) Kpa-clawbot/CoreScope#1367 chat-app layout) builds name, time and preview but has no unread badge.
• The live update path for mobile rows (around line 1831) should keep the badge in sync too.

Proposed fix

Render the same badge in the mobile row (e.g. in .ch-row-line1 next to the time, or on the avatar), with the same text (99+ cap), title and aria-label, escaped as on desktop and styled with existing CSS variables. Clear it when the channel is opened, as on desktop.

Acceptance criteria

• A channel with unread > 0 shows the badge at mobile width (e.g. 390 px); none at 0.
• The badge updates when a live-decrypted message arrives for a channel that is not open, and clears when it is opened.
• Unit test for the mobile renderer and a Playwright check at mobile width; desktop unchanged.