cli: html output: valuation-carrying register links, and tree depth classes

[acinader]

Two small additions hledger's HTML output

Part of a prototype I am working on (https://github.com/acinader/hledger/tree/web-playground, cf #2768)
they live in the hledger package and are useful on their own:

• With --base-url, register links carry a value parameter naming the
  report's valuation, mirroring --value's WHEN[,COMM] syntax
  (value=cost, end, end,€, then, now, a date). A register that
  converts amounts the same way ends on the figure that was clicked —
  hledger-web's register will use this; anything else ignores
  it. Cost combined with a valuation emits none, as before.
• In tree mode, html account cells carry their tree depth as a class
  (account depth-N), so a stylesheet can indent the tree and a UI can
  fold it. List mode and other formats are unchanged.

Both are inert until a consumer uses them; the consumer is the
web-playground branch above, and #2768's refactoring.

Test plan:

$ hledger -f examples/sample.journal bal -M -V -O html --base-url '' | grep -o 'value=end' | head -1
value=end
$ hledger -f examples/sample.journal bal -M -B -O html --base-url '' | grep -o 'value=cost' | head -1
value=cost
$ hledger -f examples/sample.journal bal -M -B -V -O html --base-url '' | grep -c 'value='
0
$ hledger -f examples/sample.journal bal --tree -O html | grep -o 'class="account depth-[0-9]"' | sort -u
class="account depth-0"
class="account depth-1"
$ hledger -f examples/sample.journal bal -O html | grep -c depth-
0

Functional tests: hledger/test/balance/base-url.test gains cases 23–24,
and a new hledger/test/balance/html.test pins the depth classes both
ways.

AI usage: Claude (Opus 5.5 and Fable 5), ~5k output tokens.