Improve block clock rendering and sync state

[johnny9]

Summary

This PR fixes how the block clock represents sync/catch-up state and how confirmation history renders in both the full and mini clocks.

• Keep the UI in sync mode until Core has left IBD and both headers and the active chain have caught up. Near-tip catch-up remains at 99.9% instead of prematurely showing the normal clock.
• Render rounded confirmation segments without losing confirmation depth when closely spaced block boundaries coalesce, and use one continuous gradient across combined blocks.
• Correct the mini block clock gradient so clockwise time moves from older, higher-confirmation yellow toward newer, zero-confirmation red.
• Move timeline ownership from ChainModel to BlockClockModel and stop presentation work while the clock is not visible. Hidden pages or tabs—and hidden or minimized windows—do not schedule dial repaints or run dial, peer-indicator, or estimating animations. The backing model stays current with minute-aligned clock updates and an immediate refresh when a block arrives, so the clock renders the latest state when it becomes visible again.

Visual evidence

Catch-up sync-state bug

The unfixed build briefly shows the normal red clock while the active chain is still rapidly catching up from a stale signet tip. The new sync-state latch keeps the UI in syncing state at 99.9% until both header and block catch-up are complete.

Screencast.from.2026-07-19.17-37-22.mp4

Confirmation segment rendering

Before	After
Every block was drawn individually. When several boundaries landed only a few pixels apart, the segments crowded together and their confirmation-color changes became difficult to see.	Rounded segments coalesce when necessary while preserving confirmation depth and a continuous gradient.

Mini block clock gradient

The mini clock now uses the same direction as the full clock: older, higher-confirmation time is yellow, transitioning clockwise toward the newest, zero-confirmation time in red.

Motivation

verificationProgress > 0.999 can report the normal clock too early. With a saved signet tip at height 313768, Core had already left IBD at 0.999830; once a peer announced height 313904, the UI showed a normal clock while 136 blocks rapidly caught up.

The existing timeline also mixed model data with paint geometry, kept repaint requests, timers, and animations active while views were hidden, and lost confirmation transitions when rounded segments were too close to render separately.

Related issues

Fixes #227.
Fixes #743.
Fixes #785.

Related to #576.
Follow-up to #609.

Testing

• Built bitcoin-core-app, bitcoinqml_unit_tests, and bitcoinqml_qmltests.
• Ran QT_QPA_PLATFORM=offscreen QT_QUICK_BACKEND=software LIBGL_ALWAYS_SOFTWARE=1 ctest --test-dir build --output-on-failure (2/2 passed).
• Built each commit in the branch independently.
• Ran Bitcoin lint checks for commit messages, tabs/whitespace, and trailing newlines.
• Ran git diff --check.
• Verified visibility gating in the C++ and QML tests: inactive clocks stop dial timers, estimating animation, peer-indicator animation, and repaint scheduling while model state remains current for reactivation.
• Added BlockClockModel coverage for minute alignment and block-triggered time refresh (BlockClockModelTests: 10/10 passed), and reran the QML suite (389/389 passed).
• Manually reproduced stale-signet catch-up with the unfixed and fixed builds.