Exploratory: Weighted buckets manual mode - #629

SDVX has whole lvls up to 17, then 17.5, then 0.1 steps from 18.0 to
20.9, with no `granularTierResolution` set. The manual editor derived
its step size from that field, so it fell back to 1 and the spinner
jumped 19 -> 20, skipping the entire fractional range.

- Derive the step from the smallest gap between adjacent available lvls
  instead, which covers granular-tier games and inherently fractional
  ones alike.
- Snap a typed bound onto a lvl the game actually has, on blur. Buckets
  match on a plain range so an off-lvl bound was never wrong, but it
  made the bucket's real extent hard to read and accumulated float noise
  while stepping. Replaces `clampValueOnBlur`, since a snapped value is
  always in range.
- Show a bucket's literal bounds as a tooltip in the by-lvl-range editor
  when they differ from its label: a bucket labelled "18" covers
  18.0-18.9 on SDVX, which was invisible before.

maimai stores chart constants (13.7) rather than the lvls players use
("13+"), and the app rendered the raw number everywhere — on cards, in
Lvl Min/Max, and in the new bucket editor. A "+" lvl is a span of
constants, not one value, so this needs more than a display swap.

- New `src/lvl-display.ts` builds a list of `LvlBand`s per game: the
  label a player uses plus the range of internal values behind it. Games
  whose lvls map one to one get single-value bands, so they're unchanged.
  Driven by a new optional `meta.lvlPlusThreshold` rather than hardcoded
  per-game logic; maimai sets 0.6.
- `LvlBoundInput` renders and parses bounds in that notation. Which end
  of a band a bound resolves to depends on whether it opens or closes a
  range, so a bucket from "13" to "13+" covers 13.0 through 13.9 the way
  a player expects. Arrow keys step band to band, Enter commits, Escape
  reverts, and unparseable text is flagged instead of coerced.
- Lvl Min/Max and song cards use the same notation, so a lvl reads the
  same everywhere in the app.
- Tier-drawing games label bounds "T07", matching how the by-lvl-range
  editor already showed them.

Buckets are still plain `{low, high}` numbers — none of this reaches the
draw logic, which continues to match on raw values.

Two fixes found while testing this:

- Escape inside a lvl input closed the entire settings drawer, since the
  keydown propagated up to the Drawer. Cancelling an edit now stays local.
- Cards on tier-drawing games briefly rendered a mix of "T11" and "13":
  bound picking enumerates tiers while a card shows the chart's own lvl,
  and those are different numeric spaces. `getChartLvlBands` keeps them
  apart.

This supersedes the SDVX step-size derivation from the previous commit;
bounds no longer step by a raw increment at all, they step band to band.

This reverts commit 9038266.

maimai's plus lvls are a display convention in the game, but the decimal
chart constants behind them are meaningful in their own right and are
what the app should show and accept. Collapsing 13.0-13.9 into "13" and
"13+" threw that detail away.

So maimai wants the same handling as any other game with non-integer
lvls: raw values, stepped and snapped by the data's own increment. That
is what the preceding commit already does, generically, via the smallest
gap between adjacent available lvls — for maimai that comes out to 0.1,
the same as SDVX.

Reverting takes the two fixes in that commit with it, but both were
fixes to problems it introduced: Escape closing the settings drawer came
from the keydown handling it added, and the T11/13 mixup on tier-game
cards came from routing card lvls through the bound-picking band list.
Neither exists without it.

Also drops `meta.lvlPlusThreshold` from the schema, maimai's data file,
and the maimai import script, since nothing reads it now.

Turning on granular lvls left hand-authored buckets holding whole-lvl
bounds while chart matching switched to sanbai tiers, so a bucket of
"13 to 13" went from every lvl 13 chart to only the few sitting exactly
on 13.00. The by-lvl-range layout already redraws itself across that
toggle; manual buckets now get the same treatment and land on identical
spans (13.00-13.95 on DDR World), because both go through the same
`topOfWholeLvl` helper rather than reimplementing the edge.

Toggling back collapses bounds to whole lvls again, so a whole-lvl
bucket round-trips unchanged.

Adding a bucket now spans the whole lvl it opens on for the same reason:
in granular mode a fresh single-tier bucket matches almost nothing. On
games with inherently fractional lvls this also improves the default —
adding after a bucket ending at 19.4 gives 19.5-19.9 instead of
19.5-19.5.

Pool sizes dip slightly with granular lvls on (304 -> 303 for lvl 13),
which is the pre-existing rule that granular mode skips charts with no
sanbai tier, not an artifact of the rescale.