imp: web: preserve sidebar position during navigation

[acinader]

This is an exploratory draft PR for hledger 2.x main.

When reviewing a large chart of accounts in hledger-web, clicking account links currently reloads the full page and resets the account sidebar position. This patch keeps the existing server-rendered links and progressively enhances same-origin /journal and /register navigation when JavaScript is available:

• keep ordinary hrefs for no-JS / failed-JS fallback behavior
• fetch the target page as HTML
• replace only #main-content
• refresh the sidebar account table so the active account highlight updates
• preserve the existing sidebar container and scrollTop
• use history.pushState / popstate for URLs and browser back/forward

I also made #sidebar-menu a bounded vertical scroll container.

Related context:

• Usability improvement proposals for hledger-web #200 includes "Both sidebar and entries should have their own scroll bars."
• it should be possible to use hledger-web with javascript disabled #33 says hledger-web should remain usable with JavaScript disabled; this keeps normal links as the fallback.
• web: split up into REST api server and a js client #316 was a larger REST API / JavaScript client idea; this intentionally avoids that scope.

Checks run locally:

• node -c hledger-web/static/hledger.js
• git diff --check
• cabal v2-build hledger-web --enable-tests --allow-newer --constraint='persistent >=2.17'
• cabal v2-test hledger-web:apptest --allow-newer --constraint='persistent >=2.17'

The Cabal build/test needed the persistent >=2.17 constraint because this machine only has GHC 9.14.1, and the default plan selected persistent-2.11.0.4, which did not compile with this toolchain. With the local dependency workaround, hledger-web built and its test suite passed: 4 examples, 0 failures.

Manual smoke check:

• Ran the patched hledger-web against a generated local journal with 81 accounts.
• Confirmed the patched hledger.js and hledger.css were served.
• Manually tried sidebar account navigation in the in-app browser and found it worked well in first use.

Still worth checking before considering this ready:

• more real-world use on larger account trees
• browser back/forward behavior
• register chart rendering after AJAX navigation
• add-transaction modal behavior when hledger-web is not in view-only mode
• sidebar hidden/shown cookie behavior
• behavior with a non-root --base-url

AI assistance disclosure:

This patch and PR text were prepared with OpenAI/Codex assistance. Approximate usage: several interactive Codex turns for research, patch drafting/refinement, local testing, and PR preparation. I understand the project currently prefers not to accept OpenAI-assisted work; I am opening this as a draft for review/discussion and to learn whether this direction is useful.

[acinader]

I have been using it for a few weeks now.