feat(dashboard): channel details page — closes #496

[kitWarse]

Closes #496

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Summary

Implements issue #496 — Create a dedicated page showing channel details, subscriber count, and notification statistics.

New file

dashboard/src/pages/ChannelDetailsPage.tsx

• Summary strip — aggregate stats across all channels: total channels, subscribers, notifications sent, overall delivery rate, active channel count
• Channel cards (one per channel: In-App, Email, Discord, Telegram) showing:
  • Icon, channel name, protocol tag (e.g. SMTP / Sendgrid)
  • Status badge — Active / Degraded / Inactive with colour-coded dot indicator
  • Channel description
  • Subscriber count highlighted in the channel's brand colour
  • Delivery rate progress bar (gradient fill, CSS transition)
  • 4-column stats grid: Sent · Delivered · Failed (with failure %) · Pending
• Skeleton loaders with shimmer animation while data loads
• Error banner with a Retry button
• Data loaded via a simulated async fetch — structured to be swapped for a real endpoint

Modified files

dashboard/src/App.tsx — imports ChannelDetailsPage, adds 'channels' to the Tab type, adds a Channel Details nav button, renders the page

dashboard/src/index.css — full .cd-* stylesheet appended:

• Responsive grid: 2-col at 900 px, single-col at 600 px
• Stats grid collapses from 4-col → 2-col on narrow screens
• Hover lift effect on channel cards

Acceptance Criteria

Criterion	Status
Channel details are displayed	✅ Icon, name, protocol, description, status, subscriber count
Statistics load correctly	✅ Skeleton → data transition; error + retry path covered
Responsive layout is maintained	✅ Breakpoints at 900 px and 600 px

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