feat: web service worker + push subscription flow (#240)

[Tijesunimi004]

What

Implements the full client-side push notification flow in apps/web per #240.

Files

apps/web/public/sw.js (new)

Plain-JS service worker served at /sw.js. Three event handlers:

• install → skipWaiting() so updates activate immediately.
• activate → clients.claim() so the SW controls all open tabs.
• push → shows a content-free notification. The body is always "You have a new message"; no message ciphertext is ever shown. Tagged by conversationId so repeated pushes for the same thread collapse rather than stack.
• notificationclick → closes the notification, then either posts sw:sync to an already-open window and calls client.focus(), or opens a new tab at /app/conversations/{id}. Never calls client.navigate() (only valid on controlled clients).

apps/web/src/hooks/usePushSubscription.ts (new)

usePushSubscription(token) hook:

1. Registers /sw.js on mount (guarded for SSR, no serviceWorker, and no PushManager).
2. On mount, if permission is already granted, re-POSTs any existing subscription to keep the server in sync.
3. Exposes requestSubscription() — the only path that calls Notification.requestPermission(). Called only when the user explicitly clicks "Enable". Never called eagerly.
4. On grant: subscribes with NEXT_PUBLIC_VAPID_PUBLIC_KEY, then POST /push/subscriptions with Authorization: Bearer <token>.

apps/web/src/components/PushPermissionPrompt.tsx (new)

Dismissible banner:

• Waits 5 s after mount before appearing (contextual — not on first load).
• Hidden when Notification.permission is granted or denied, when already subscribed, or when dismissed.
• Dismissal recorded in sessionStorage so it stays hidden for the session.
• "Enable" button calls requestSubscription(); "Not now" hides for the session.

apps/web/src/app/app/layout.tsx (modified)

• Mounts <PushPermissionPrompt /> at the bottom of the authenticated shell.
• Adds a navigator.serviceWorker message listener that handles sw:sync events from the SW's notificationclick handler, navigating the Next.js router to the relevant conversation (triggering a data sync).

Acceptance criteria

• SW registers + subscribes after user grants permission: usePushSubscription registers the SW on mount and subscribes only when requestSubscription() is called (after the user clicks "Enable" in the prompt).
• Subscription persisted server-side: postSubscription() POSTs the PushSubscription JSON to POST /push/subscriptions with the user's JWT.
• Clicking the notification opens the conversation and syncs: the SW's notificationclick posts sw:sync to the open client; the layout's message listener navigates to /app/conversations/{id}, which triggers the page's data fetching hooks.

Environment variable required

NEXT_PUBLIC_VAPID_PUBLIC_KEY=<base64url-encoded VAPID public key>

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Hey @Tijesunimi004! 👋 It looks like this PR isn't linked to any issue.

If this PR is for one of the issues assigned to you as part of a Wave, please link it to ensure your contribution is tracked properly. You can do this by adding a keyword to the PR description (e.g., Closes #123), or by clicking a button below:

Issue	Title
#240	Web service worker + subscription flow (apps/web)	Link to this issue
#194	Delivery pipeline: validate → persist → resolve devices → push	Link to this issue

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