feat: feat: sync selected campaign to URL query param

[simplicityf]

Add URL-based campaign selection

Selected campaigns are now reflected in the URL as ?campaign=<id>, enabling direct links to any campaign on the board.

Changes

App.tsx

• Added getCampaignIdFromUrl() to read ?campaign= from window.location.search on mount
• Added setCampaignIdInUrl() using history.replaceState to keep the URL in sync without polluting browser history
• Bootstrap effect now validates the URL param against the fetched campaign list — restores a valid ID, falls back to the first campaign and shows a warning banner for an invalid one, or defaults to existing behaviour when no param is present
• Added invalidUrlCampaignId state and a dismissible amber warning banner rendered above the metric grid with role="alert"
• Replaced direct setSelectedCampaignId in onSelect with a handleSelect wrapper that also clears the invalid-link banner on intentional navigation

invalid-campaign-banner.css (new)

• Amber warning banner styles; add to your main stylesheet

Behaviour

Scenario	Result
?campaign=42 — valid ID	Campaign 42 selected and detail panel loaded
?campaign=99 — unknown ID	Warning banner shown, falls back to first campaign
No param	Default behaviour unchanged (first campaign selected)
User selects a campaign	URL updated in place; banner dismissed if visible

Notes

• Uses replaceState not pushState — selecting campaigns doesn't pollute browser history, but the param is visible in the address bar for copying/sharing
• No router library added; plain URLSearchParams + history.replaceState covers the requirement with zero new dependencies

Related Issue

Closes #8

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