Add Threads settings and standardize visual bells

[brsbl]

What was wrong

Thread-specific preferences were mixed into General settings, and archived conversations had no user-controlled retention policy. Visual bells also lacked one consistent layout for wrapping, actions, loading state, and dismissal.

What changed

• Adds the real Settings → Threads page and moves the three thread-only controls out of General.
• Adds Archived conversations: Keep forever (default) or Delete after 30 days.
• Applies the 30-day policy only to threads archived after the setting is selected. Unarchiving cancels the scheduled conversation deletion; existing archives are not backfilled.
• Adds matching REST, SDK, CLI (bb settings threads), and discoverable settings documentation.
• Standardizes production visual bells around non-wrapping rows, a visible dismiss control, low-emphasis underlined actions, and the shared Loading glyph.
• Changes the archived-thread bell to a Thread Archived title row followed by the clickable, non-underlined thread name and Undo on one row.
• Updates the existing Settings and Toast Catalog stories and focused UI tests.

This layer owns the Threads settings, archived-conversation retention, and visual-bell changes. It does not implement thread-storage, linked-worktree, or managed-environment cleanup; those changes remain isolated in the child PR.

The actionable archive bell keeps the existing 10-second duration.

Before and after

Settings before — PR merge base at 2d7850c030b5dffb428d2fa498ea1b94bb34c886

The /settings/threads route fell back to General, where the thread controls were mixed with app-wide settings.

Settings after — PR head at c69ea7f561a9e407c6950bdb1aef51129a677632

Both Settings images use the exact compared revisions, the same fresh dev fixture and /settings/threads route, Chrome for Testing 151.0.7922.71, and a 1440×900 viewport.

Archived-thread bell before — PR merge base at 2d7850c030b5dffb428d2fa498ea1b94bb34c886

Archived-thread bell after — PR head at c69ea7f561a9e407c6950bdb1aef51129a677632

Both bell images use the exact compared revisions and the same Toast Catalog fixture in Chrome for Testing 151.0.7922.71 at a 1440×900 viewport.

How you verified

• All required remote CI checks passed on c69ea7f561a9e407c6950bdb1aef51129a677632.
• Exact-revision branch web apps render the prior General page and the new Threads page at /settings/threads without runtime errors.
• Exact-revision Toast Catalog fixtures render the prior and standardized archive bell states.
• git diff --check passed for the final layer diff.

BB-Thread-ID: thr_q34zuky3js

AGENT GENERATED