Add Classic Icons toggle to Graphics Settings

Adds a "Classic icons" toggle in the structure-icons section of the
Graphics Settings modal. Off (default) keeps today's renderer look;
on switches to a classic style — lighter player-colored shape behind
a dark icon glyph, with 0.75 alpha for a subtle translucent feel.

Exposes the underlying tuning as new render-settings knobs
(`structure.fillDarken`, `borderDarken`, `iconAlpha`, `iconR/G/B`) and
threads them through the structure shader as uniforms, replacing the
previously hardcoded `darken(_, 0.65)` / `darken(_, 0.35)` calls and
the hardcoded white `vec3(1.0)` icon color. The `classicIcons` boolean
in the override schema is the single user-facing knob; the generator
derives the five underlying field values from it. Extends the
ClientGameRunner live-apply path to copy the `structure` slice too,
and adds tests covering the schema and preset derivation.