refine(desktop): surface unread thread cues

[daveh-beep]

Summary

Makes unread thread activity visible inside the source channel without adding navigation or state-machine behavior:

• Promotes the thread summary’s muted unread text into a legible N new pill.
• Adds a continuous accent spanning the parent message and its thread summary.
• Includes the unread count in the thread summary’s accessible name.

Why this slice

This is the smallest durable response to the in-channel dead end: users who enter a channel directly can see which parent holds unread replies. It changes only presentation over the existing per-thread unread count and read frontier—no new route, jump behavior, dismissal state, or server data.

The earlier floating jump action was removed after red-team review found multi-thread dismissal and virtualizer edge cases that would require a larger state model. That work can return independently if needed.

Verification

• Diff rebuilt from current main
• git diff --check
• TypeScript typecheck
• Biome
• File-size and px-text guards
• 3,885 desktop unit tests passed
• 13 targeted thread-unread Playwright smoke tests passed

[daveh-beep]

🤖 ## Corrected design scope — in-channel unread-thread cues

Rendered from the corrected PR head (9eb8c616; based on current main).

The red-team pass removed the floating jump pill: its channel-wide dismissal state could hide a second unread thread, it targeted the deferred rather than rendered virtualizer model, and it could toggle an already-open thread closed. Those behaviors were too risky for this low-diff slice.

This PR now changes only durable presentation over existing unread state:

• a continuous accent spans the parent message and thread summary;
• the existing muted unread text becomes a legible N new pill;
• the thread summary's accessible name includes the unread count.

No navigation, dismissal state, routing, server data, or sidebar behavior is included.

[klopez4212]

I think the pill is cool and stands out more..might still ask wes to review because I have no idea what breaks things

[daveh-beep]

🤖 Closing this stale slice so we can reset to current main and re-propose only the smallest still-useful change locally first. This branch has diverged from the current unread/notification work; preserving the rationale here rather than carrying conflicts forward.