fix(ui): take every corner radius from the theme's radius tokens

[Tryanks]

Changed behaviour

Corner radii came from three places none of which was the theme's tokens: a hard-coded scale in material.rs (radius_overlay() = 10, radius_input() = 8, …), a flat theme.radius field, and 42 literal rounded(px(N)) values. The theme filled gpui-base's RadiusTokens with its own formula, and nothing in Tcode read them.

Now:

• theme.rs derives the tokens the way gpui-kit derives them from the same two theme-file keys (radius → md, radius.lg → lg, sm = radius/2, xl = 2×radius, full = pill). radius.lg was in themes/tcode.json already and unread. The flat radius field is removed; the scrollbar thumb uses full.
• The material::radius_* helpers take cx and map a role to a token: overlay/composer → lg, card/input/button → md, chip → sm, bottom sheet → xl. overlay_popover and semantic_chip take cx for the same reason.
• Literal radii map to the nearest token (12 → lg, 10/8 → md, 6/4 → sm); 1–3px hairline details stay literal.

With the theme's radius 10 / radius.lg 14 the visible changes are small: inputs and buttons 8 → 10, chips and small rows 6 → 5, the phone sheet 16 → 20.

Per Principle 7 (#564); closes the "radius scale" item left open after the gpui-base audit.

Tests

No new tests: this is theme wiring. cargo nextest run -p tcode-ui --locked — 349 passed.

Checks run

• cargo fmt --all --check, cargo clippy --workspace --all-targets --locked -- -D warnings
• Visual: desktop app (throwaway profile) in light and dark — thread view with composer and terminal drawer, model picker popover, settings page. Phone sheet not re-checked (its radius helper changed from 16 to 20 only).