add runtime-selectable icon themes (nerd/unicode/ascii)

[roykollensvendsen]

Problem

taolk hard-coded Nerd Font glyphs with no fallback. On any terminal without a patched Nerd Font the icons show as blank boxes ("tofu"), and in GNOME Terminal/VTE the symbols-only fallback font renders them about 1.6 cells wide, which breaks the panel borders. Users had to install and set up a Nerd Font just to get a readable UI.

What this does

Makes the icon set runtime-selectable with three themes:

• unicode (new default) — plain single-cell symbols that render on any terminal, no special font
• nerd — the original Nerd Font glyphs (opt-in; needs a patched font)
• ascii — pure ASCII for the most limited terminals

Chosen via the ui.icons config key or the TAOLK_ICONS env var; NERD_FONT=1 is honored as a hint; non-TTY output falls back to ascii. The nerd theme reproduces the current look exactly.

How it works

An IconSet struct with three const instances and a process-global accessor (OnceLock) resolved once at startup. The compile-time tables (command palette, help cards, hint bar) reference an Icon enum that resolves through the active theme at render time. Resolution is a pure function (resolve_with), so it's unit-tested without touching env or IO.

Keeping it terminal-agnostic

• A test asserts every glyph in every theme is exactly one display cell and a single Unicode scalar, so a wide or ambiguous glyph can't slip in.
• A test asserts no Nerd Font PUA glyph appears anywhere outside the icon module, so icons can't be scattered as raw literals again.
• The unicode glyphs were picked from East-Asian-Width "narrow" ranges so they stay single-cell even under CJK locales.

Testing

cargo fmt --check, cargo clippy --all-targets -- -D warnings, cargo test (507 passing), and cargo check --no-default-features --lib all pass. Also checked ui.icons / TAOLK_ICONS selection and the non-TTY fallback by hand.

Companion to #13: with a safe default, a Nerd Font is now optional rather than required.