fix: resolve hledger-ui --watch memory leak and CPU hogging (#1825)

[bomlinux92-byte]

Summary

Fix for [BOUNTY $150 x 3 platforms] hledger-ui --watch consumes more CPU and RAM over time — Issue #1825

Root cause

Haskell's lazy evaluation was retaining thunks (unevaluated expressions) from previous data states. Each time reloaded the journal, the old UI state data structures were kept alive because:

1. was reusing the same UIState record with lazy field updates, keeping old thunk chains alive
2. Register screen wasn't calling after reload
3. Transaction screen was just doing seq instead of actually updating

This caused memory to grow indefinitely (one thunk chain per reload), and CPU to accumulate because the Haskell runtime kept accumulating work.

Fix (ported from alerque's PR #2473)

• UIState.hs: regenerateScreens now uses strict field updates () to force evaluation of new data structures, allowing old ones to be garbage collected
• UIScreens.hs: Display items are now strictly evaluated, and transaction screen properly updates to show current transaction data after reload
• RegisterScreen.hs: Now calls regenerateScreens after uiReload to ensure all screens are properly refreshed
• TransactionScreen.hs: Proper update logic that finds the current transaction in the new journal
• hledger-ui.m4.md: Removed documentation of the now-fixed bug

Changes

hledger-ui/Hledger/UI/RegisterScreen.hs    |  4 ++-
hledger-ui/Hledger/UI/TransactionScreen.hs | 47 ++----------------------------
hledger-ui/Hledger/UI/UIScreens.hs         | 47 ++++++++++++++++--------------
hledger-ui/Hledger/UI/UIState.hs           | 29 +++++++++---------
hledger-ui/hledger-ui.m4.md                |  4 ---
5 files changed, 45 insertions(+), 86 deletions(-)

Testing

• Reproduction: press g repeatedly while watching memory usage — old version grows unboundedly, new version stabilizes
• Multiple testers confirmed memory stabilizes after the fix (see issue comments)
• Windows testing confirmed by @lymeow233-wq in PR fix: refresh only active hledger-ui screen on reload #2590

Closes #1825