Reset timer [Loading] and usage-error placeholders cannot be hidden

[zachthedev]

Problem

Block Reset Timer and Weekly Reset Timer render a [Loading] placeholder and the
[No credentials] family of usage errors with no way to turn either off:

• src/widgets/BlockResetTimer.ts:136-142
• src/widgets/WeeklyResetTimer.ts:220-226

WeeklyUsage.ts:86-90 answers the identical condition, absent usage data, by
returning null so the widget disappears, and the three usage widgets have a
hide state for it. So a status line that goes quiet when usage data is
unavailable still shows [Loading] from these two.

aae5add widened this: it turned a return null into a persistent placeholder, so
the widgets went from self-hiding to always-on with nothing to get the old
behavior back.

Proposal

Both widgets declare the shared USAGE_NO_DATA_HIDEABLE_STATE their usage
siblings already use, gating the error text and the [Loading] placeholder
together. Opt-in, so the default stays exactly what it is now.

This builds on #430, which is where the shared (h)ide… checklist lives.

#542 has it, stacked on #430. It also moves two keybinds on these widgets off
h, since h opens the hide checklist there and would otherwise leave the new
state unreachable in date mode.