Extract shared fund CSS into Omni/Fund/Web/Style.hs

Extract shared fund CSS from Overview.hs and Invest.hs into a new module Omni/Fund/Web/Style.hs using Clay.

Currently both files define .fund-kpi-grid, .fund-kpi, .fund-section-block, .fund-section-title-h2, .fund-section-desc, etc. — with slight differences (Overview has uppercase labels + 14px padding, Invest has 12px). This causes visual inconsistency and maintenance burden.

Scope

Move these CSS class families into Clay definitions in the new module:

• fund-section-block, fund-section-title-h2, fund-section-desc
• fund-kpi-grid, fund-kpi, fund-kpi-label, fund-kpi-value, fund-kpi-note
• fund-kpi-val-pos, fund-kpi-val-neg
• fund-kpi-has-tip, fund-kpi-tip (tooltip pattern from Overview)
• kel-slider, kel-slider-wrap, kel-slider-row, kel-slider-label, kel-slider-val

Export as composable Clay values that can be included in a page's stylesheet. Update Overview.hs and Invest.hs to import from the shared module.

Acceptance criteria

• Both pages render identically to current (visual regression check via DevBrowser)
• No duplicate CSS definitions between the two files
• bild Omni/Web.hs passes
• lint passes