feat(competitions): list and grid view toggle on browse

[AnayDhawan]

Blocked by #196.

A list/grid switch on /competitions. List is the dense deadline-scanning view; grid is the browsing view.

Files

• src/components/competitions/view-toggle.tsx - new, 'use client'
• src/components/competitions/competitions-browser.tsx - consume it
• src/components/competitions/competition-card.tsx - support both variants

Nothing like this exists in the repo yet

Grep for viewMode|view_mode|LayoutGrid|toggle.*view across src/ returns zero hits. The nearest relatives:

• src/components/docs/docs-index-list.tsx:8-12 - its comment records that a vertical list replaced an icon-card grid
• src/components/map/results-list.tsx:29 - a generic toggle?: { label, onClick } prop used for "Show all"

Build it once here properly, because #205 needs the same control for the docs index and should reuse this component rather than growing a second one.

Behaviour

• Two-state segmented control, list and grid, with aria-pressed and a visible label or a title, not an unlabelled icon pair
• Default is list: deadlines are the reason people come to this page and a list scans faster
• Persist in localStorage under a namespaced key, read it in an effect so the server render is deterministic and hydration stays clean
• Wrap the read in try/catch; localStorage throws outright in some privacy modes

Card variants

List: full width, description clamped to three lines, four meta pills in a row, countdown on the right.

Grid: sm:grid-cols-2 lg:grid-cols-3, description clamped to two lines, meta pills wrapping, countdown under the title. Cards in a row must be equal height, so the save row pins to the bottom.

Same component with a variant prop, not two components that drift apart.

Acceptance criteria

• Toggle switches the layout with no full page reload
• Choice survives a reload and a navigation away and back
• Private browsing with localStorage blocked: defaults to list, no console error
• No hydration warning
• Keyboard operable, focus visible, state announced via aria-pressed
• Grid cards in a row are equal height at every breakpoint
• Both variants readable at 360px