feat(navigation): add deep linking support

[menawar]

Implements deep linking for SubTrackr with the custom subtrackr:// scheme and HTTPS universal-link support.

Supported URLs:

• subtrackr:// -> Home
• subtrackr://subscription/:id -> SubscriptionDetail
• subtrackr://subscription/add -> AddSubscription
• subtrackr://subscription/add?name=...&amount=...&cycle=... -> AddSubscription prefill
• subtrackr://settings -> Settings
• https://subtrackr.app/subscription/:id -> SubscriptionDetail
• https://subtrackr.app/subscription/add -> AddSubscription

closes #73

Notes:

• Adds input validation and sanitisation for deep-link params before they reach screens.
• Adds a NotFound screen for malformed or unmatched URLs.
• Adds a share-link utility for subscription links.
• Universal links require apple-app-site-association and assetlinks.json to be hosted at the real domain root before they work in production.

Verification:

• Targeted deep-link Jest tests pass.
• TypeScript passes for the touched deep-link files.
• Full repo test/typecheck still has unrelated pre-existing failures outside this issue.

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