chore(frontend): UI polish backlog from the 2026-08 UX review

[cristim]

Grouped P3 polish + accessibility items from the review (each a checkbox):

• Summary-card grid uses 3-per-row, leaving an orphan 4th card + dead space (Home, Opportunities); use a balanced responsive grid. Wide screens also underuse horizontal space beside the fixed left rail. (2.7)
• Per-service chart shows service names on the x-axis AND a redundant "Service" axis title. (Home P3)
• Opportunities "AWS lookback" control reads as global above a multi-provider table; add an inline "AWS only" hint (it only affects AWS). (Opportunities P3)
• Admin "Reset to Defaults" sits next to "Save Settings" with no confirmation; add a confirm dialog. (Admin P3)
• Verify low-contrast muted/placeholder grey text against WCAG AA. (§4)
• Verify keyboard access + visible focus states for modals (New Plan) and the Opportunities select-to-act table interaction. (§4)

Source: docs/ui-ux-review.md (2026-08 UX review) (P3 items + §4 accessibility)