fix(desktop): bound overlay scrollbars

[Areson]

Why

Thread and main-timeline scrollbar thumbs can move behind the composer, making them impossible to grab, and dragging a small native thumb can select message text instead of scrolling.

What

• Add a bounded overlay scrollbar for thread panels and main timelines, including virtualized timelines
• Reserve the composer height, enforce a minimum grab target, and map pointer drags across the full scroll range
• Reveal the thumb when the pointer enters anywhere in the scrollable pane, matching the sidebar behavior while retaining scroll-time visibility and idle fade
• Coordinate text-selection suppression across overlapping drags and restore it safely on release or cleanup
• Preserve the existing mouse cursor and avoid observing Virtua's inner measurement container so prepends retain their scroll anchor
• Add focused regression coverage for bounded geometry, drag clamping, concurrent selection-lock lifetimes, and pane-hover visibility

Risk Assessment

Low to moderate — this is limited to desktop message scrolling and does not change message data or relay behavior. Reviewers should focus on pointer capture across mouse, pen, and touch hardware, pane-hover visibility, and composer resizing.

Validation

• just ci
• Pre-push hooks: branch skew, organization policy, desktop checks/tests, Rust tests, Tauri tests, and mobile tests
• Focused pane-hover browser regression: pass
• Scroll-history and cascading-virtualization smoke regressions: 4/4 pass across two repetitions

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