feat(portal): make app mobile responsive

[keithfawcett]

Makes the portal (app.openpartner.dev) mobile responsive. It was previously desktop-only — pure inline CSS-in-JS, a fixed 248px sidebar, and no media queries anywhere.

Changes

Foundation

• useMediaQuery / useIsMobile hook (768px breakpoint) — how components branch inline styles by viewport, since there are no stylesheets to hang @media on.

Navigation → mobile drawer

• Both shells (Shell in App.tsx and CreatorShell) collapse the fixed sidebar into a sticky hamburger top bar + slide-in drawer with a tap-to-dismiss backdrop. Sidebar takes a variant: 'sidebar' | 'drawer' prop; the drawer auto-closes on navigation. Desktop is unchanged.

Shared primitives (ui.tsx)

• Page: responsive padding, stacking header, smaller title on mobile.
• Table: reflows into stacked label/value cards on phones (first column as the row title) instead of horizontal scrolling — so all data is visible without scrolling sideways. Tablet/desktop keep the real table with a horizontal-scroll fallback.

Grid sweep

• op-grid-collapse class (global.css, !important media query) collapses ~20 hardcoded multi-column form grids to one column ≤768px. Short stat pairs (PartnerCard, creator cards) intentionally stay 2-up.

Device-testing fixes

• Webhook endpoint cards: header stacks vertically on mobile, long URLs wrap, action buttons wrap so they no longer overlap the URL.

Verification

• tsc --noEmit and vite build both pass.
• Live mobile QA done on-device (screenshots reviewed); a couple of layout refinements folded in from that pass.

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