docs(changelog): Batch C — #5464 read-only session-listing projection + #5465 pause hidden-tab polling (#5455)

CHANGELOG.md

@@ -13,6 +13,8 @@
 
 ### Fixed
 
+- **The sidebar no longer keeps polling and rebuilding itself in a hidden background tab.** The 30-second session-list poll and the 60-second relative-time refresh ran unconditionally, so a backgrounded window kept fetching `/api/sessions` and rebuilding a sidebar it couldn't show. Both now pause while the tab is hidden and refresh immediately when you return to it (each poll has its own `visibilitychange` handler), so nothing goes stale — you just stop paying for updates you can't see. Part of the response to "WebUI feels laggy." Thanks @ai-ag2026. (#5465, #5455)
+- **The sidebar session list stops holding a write-capable database handle just to read.** Building the sidebar opened a read-write SQLite connection on the live (potentially multi-GB, WAL-mode) state database and re-ran a defensive index check on every refresh — needless lock/checkpoint contention while the agent is streaming into the same database. The listing now opens the database read-only, and only touches it with a separate short-lived writable connection on the rare occasion the index is actually missing. Output is unchanged. Part of the response to "WebUI feels laggy." Thanks @ai-ag2026. (#5464, #5455)
 - **A profile whose default model lives in a large provider's picker "overflow" list no longer fails validation.** When a provider has more models than the visible picker cap (15 entries), the extras are moved into an overflow bucket that still renders in the dropdown. Profile model validation only searched the visible list, so selecting one of those overflow models as a profile default was rejected with a spurious "not available for provider" error even though it appeared (and worked) in the picker. Validation now searches both the visible and overflow buckets, matching what the dropdown shows. Thanks @WallaceWebster. (#5453)
 - **Read-only sessions (cron runs, subagent views) no longer show a broken "fork from here" affordance.** Forking is only meaningful for sessions you own, but the fork/branch controls were shown on read-only cron and subagent sessions too — clicking them just produced a confusing "Session not found" error. Those controls are now hidden for read-only sessions on the frontend, and the branch endpoint refuses a read-only source with a clear `403` instead of a misleading `404`. Thanks @rodboev. (#5449, #5439)
 - **Inline Mermaid diagrams stay readable instead of collapsing to a sliver.** The inline diagram viewer shared the lightbox's fit-to-screen scale, so on mobile (and other short viewports) a diagram could shrink to an unreadable strip. Inline rendering now uses its own width-based sizing with a minimum height floor (and its own minimum scale), decoupled from the full-screen lightbox fit, so inline diagrams keep a usable size while the lightbox zoom/pan behavior is unchanged. Thanks @rodboev. (#5434, #5413)