Retain a stable message window in channel timelines

[tlongwell-block]

Summary

• virtualize channel timelines with Virtua while preserving semantic message/day-divider keys, premeasuring rows ahead of the viewport, and retaining recently visible rows to limit remount churn
• stabilize older-history pagination with exact-suffix prepend admission, settle-gated commits, bounded late-measurement anchor correction, and upward-momentum handling at the history boundary
• keep live arrivals pinned at the tail when following the conversation, but buffer them behind the “new messages” affordance while reading history; preserve explicit search, unread, deep-link, and send navigation
• keep composer growth, dynamic row measurements, channel/window changes, and bottom settling from moving the reader’s position
• progressively render message images from NIP-92 thumbnails, then decode and cross-fade the original inside a fixed-aspect-ratio frame; keep original URLs for zoom/copy/download and avoid eagerly preloading attachment originals
• preserve learned aspect ratios for dim-less videos across virtualized remounts and retain live thread replies/reactions when an older thread page resolves in flight
• add focused unit and Playwright coverage for virtualization, prepend anchoring, live-tail buffering, media layout, progressive image request order, thread merging, reconnect backfill, and navigation behavior

Implementation notes

• adds virtua@0.49.2 plus a narrow patch so measurement updates above the viewport preserve the current visual anchor
• day dividers are standalone virtual items only after their boundary is proven, keeping prepend keys a strict suffix and Virtua’s size cache aligned
• pure history prepends wait for wheel/scroll quiet plus stable frames (with a bounded deadline); simultaneous prepend + append snapshots, including the user’s own send, pass through immediately
• timeline image warming remains available for non-attachment media, while message attachments keep native lazy loading so the thumbnail request completes before the full-resolution request starts

Validation

• required CI is green at 8171974bfd98f485531e3fbd0932351a8a49ad2d: workflow run
• Desktop Core
• Desktop Smoke E2E (all four shards)
• Desktop E2E Relay
• Desktop E2E Integration (both shards and aggregate)
• Desktop Build (macOS)
• Web and dead-token-reference guard
• local typecheck, production build, desktop unit suite, focused progressive-image/virtualization Playwright coverage, Biome, file-size guard, and desktop/mobile/Tauri push-hook suites passed

Local environment note

The unrelated local Rust database hook could not run in this workspace because its Rust 1.89 toolchain is older than the Rust 1.94 required by sqlx 0.9; all required applicable GitHub checks passed.