fix(ui): replace emoji UI controls with lucide icons (tri-state checkbox, ⚙ header, ✕ button)

[ajianaz]

Problem

Emoji are used as functional UI controls in 3 places. Emoji render differently across OSes (Apple vs Google vs Windows) — a tri-state checkbox becomes ambiguous on Windows, and a settings header carries inconsistent visual weight.

Location	Current	Why it's a problem
NamespaceFilter.svelte:126	'☑' / '☐' / '⊟' (select-all tri-state)	Tri-state via 3 different glyphs — renders inconsistently across OS; ⊟ reads as "minus in box", not "some selected"
SettingsModal.svelte:86	⚙ Settings header	Emoji gear vs lucide icons elsewhere on the same surface
ToolsView.svelte:390	✕ remove-button	Tiny hit area + not from the icon set

Fix

• NamespaceFilter: real checkbox input with indeterminate DOM property (handles tri-state natively), or lucide Check/Minus inside a styled box
• SettingsModal: lucide Settings icon (size 16, strokeWidth 1.75 — match h2 weight)
• ToolsView: lucide X (size 12–14)

All lucide already imported in these files' siblings — no new deps.

Source

Full-app UI audit (miifb #18: one icon system, SVG over emoji).