Move fast mode to a bolt button in the traits pane

[Tryanks]

What changes

Fast mode is pulled out of the traits (reasoning) picker's option list and becomes a persistent lightning-bolt button pinned to the pane's top-right corner.

• Filled amber bolt (#F5A524, one hard-coded colour in Theme::fast_mode_accent, deliberately not a theme token) = on; outline bolt in muted foreground = off. Click toggles and keeps the pane open.
• Models without fast mode still show the bolt, dimmed and inert: no hover background, cursor_default, no handler. On desktop a tooltip reads "Current model cannot enable fast mode"; in the compact layout tapping does nothing and there is no tooltip.
• The bolt wrapper is absolutely positioned above the scroll container and .occlude()s it; on_click stops propagation. Hovering or clicking the bolt never reaches an option row scrolled underneath it.
• The Claude "Fast Mode" rows are no longer listed. A Codex "Service Tier" section is rendered only when tiers other than default and fast exist, and then without the Fast row.
• FastMode in crates/core/src/provider_models.rs is the single owner of "does this model have fast mode / is it on / which selection toggles it" (Claude fastMode boolean, or Codex serviceTier select with a fast-valued or "Fast"-labelled tier, which the app-server reports as priority). The composer bolt and the Orchestrate settings Fast switch both use it; the duplicate model_fast_supported is removed.
• New assets/icons/zap.svg / zap-filled.svg (Lucide, currentColor). toolbar_icon_button now accepts any Icon.
• Locale keys composer.fast_mode and composer.fast_mode_unsupported in both en.yml and zh-CN.yml. docs/DESIGN.md composer section updated.

The chip label is untouched: the desktop chip still shows Fast/Normal or the tier name.

Tests

fast_mode_keys_on_claude_boolean_and_codex_fast_tier covers the boolean shape, the app-server select shape (default/priority), and an unsupported spec, replacing the Orchestrate settings test that exercised the same rule.

Visual verification (macOS, TCODE_DATA_DIR throwaway profile)

• Claude Opus 5: no Fast Mode section; outline bolt; click → filled amber, chip "High · 快速 · 1M", pane stays open. With the list scrolled so an option row sat under the bolt: hovering the bolt showed no hover background on the row, clicking toggled fast without selecting the row and without dismissing the pane.
• Codex GPT-6 Astra: no Service Tier section; click → filled amber, chip "Medium · Fast".
• Claude Fable 5.1: dimmed bolt; hover shows only the tooltip, no hover circle; click does nothing.
• Compact (600 px wide): bolt at top-right in a 44 pt slot; dimmed state checked with Fable.

Checks run

• cargo fmt --all --check — pass
• cargo clippy --workspace --all-targets --locked -- -D warnings — pass
• cargo build --workspace --locked — pass
• cargo test --workspace --locked — pass (core 128, ui 413, runtime 170)