Icons: replace default hardcoded color with currentColor

[talissoncosta]

Problem

~60 SVG icons in Icon.tsx use hardcoded fill colours that break dark mode and prevent CSS classes from controlling icon colour. The main offenders:

• 41x fill={fill || '#1A2634'} — dark navy, invisible on dark backgrounds
• 4x fill={fill || '#9DA4AE'} — grey, poor contrast in dark mode
• 2x fill={fill || '#656D7B'} — grey, poor contrast in dark mode
• 7x fill={fill} — no fallback, renders nothing when no fill prop passed
• 2x fill={fill ?? '#000000'} — black, invisible in dark mode
• 1x fill={fill || 'white'} (plus icon) — invisible in light mode, callers always override

Inline fill attributes always win over CSS, so utility classes like fill-body, fill-primary, and hover-color-primary can't control icon colour.

Files

• web/components/Icon.tsx — all changes in one file

Fix

Replace all hardcoded fills with fill={fill || 'currentColor'}. This inherits the text colour from the parent element, working in both light and dark mode automatically.

Icons with intentional semantic colours are left unchanged:

• nav-logo — white (sits on dark navbar)
• info — cyan (#0AADDF)
• warning — orange (#FF9F43)
• close-circle — red (#EF4D56)
• checkmark-square — purple (#6837FC)
• required — blue (#4285F4)
• google, pr-merged, pr-closed, pr-linked, issue-closed, issue-linked — brand colours baked into SVG paths

Additional findings

10 unused icons (zero references in application code): dash, diff, edit-outlined, email, layout, nav-logo, paste, required, settings-2, star. Candidates for removal in a follow-up.

Impact

• Fixes ~57 broken icons in dark mode in one go
• Highest ROI fix in the entire design system audit
• Unblocks QW-9 (SidebarLink hover) and QW-2 (hardcoded colours in components)
• After this change, CSS utility classes control icon colour without inline overrides

Acceptance Criteria

• No instances of #1A2634 or #1a2634 remain in Icon.tsx
• All generic icons use currentColor as the default fill/stroke
• Icons render correctly in light mode (inherit dark text colour)
• Icons render correctly in dark mode (inherit light text colour)
• npm run typecheck passes
• npx eslint --fix passes

Context

Part of the design system audit (#6606). See frontend/DESIGN_SYSTEM_ISSUES.md on chore/design-system-audit-6606 branch (QW-1). This is the top-priority fix — highest ROI in the entire audit.