Fix text-selection handle drag/visuals and onboarding page 1 vibrancy

[Chethan616]

Summary

• Selection handles couldn't be dragged in any direction. Root cause: the drag gesture's pointerInput was keyed on selectedOcrRanges, which the gesture itself mutates on every move. The instant a drag moved past touch slop, Compose saw the key change and cancelled + restarted the gesture coroutine mid-drag — the restarted awaitEachGesture then waited on a fresh touch-down that never came since the same finger was still pressed. Fixed by reading the live selection via rememberUpdatedState instead of keying on it.
• Handle shape redesigned to match real Android/Chrome/Google Docs handles: a circle with one sharp corner notched out exactly at the anchor point, asymmetric per side (bulges lean into the selection), replacing a symmetric "balloon on a flat neck" shape. Verified geometrically (numeric assertions + rendered PNG previews of the actual curves) before writing any Kotlin.
• Highlight band between the handles is now a fully-rounded capsule instead of a flat, barely-rounded rectangle, so it reads as one selection with the handles instead of two balloons on a rod.
• Onboarding page 1: the fanned stack of format-sheets behind the assembled book was permanently washed out (0.20/0.34 fill alpha once landed) — raised to 0.75 card / full-opacity lines, with a deliberate contrast gap kept between the two.

Test plan

• Select text and drag each handle in all four directions repeatedly — should never stall.
• Pinch-zoom with one finger already selecting text — second finger should cleanly cancel the selection.
• Compare the new handle/highlight look against stock Android text selection (Chrome, Google Docs) for proportion.
• Step through onboarding page 1 — the sheets fanned behind the book should read as vivid, distinct colors, not pale/grey.