feat: add route-driven chat navigation

[shakkernerd]

Summary

• Migrates the app to SvelteKit route-backed chat navigation.
• Adds canonical public route IDs for workspaces, channels, DMs, and thread roots.
• Supports legacy internal-ID links and canonicalizes them to public route URLs.
• Keeps SvelteKit responsible for URL parsing while ChatApp syncs state from route params.
• Fixes chat scrolling/unread behavior around live messages, unread dividers, active-channel clicks, and first-workspace onboarding.
• Adds one-time route ID backfill gating so large existing SQLite DBs do not rescan route IDs on every startup.
• Updates embedded web assets and docs for the SvelteKit build/output flow.

Verification

• pnpm check
• pnpm test:e2e passing: 32/32
• pnpm build
• docker build -t clickclack:ci .
• Targeted SQLite route ID tests
• Targeted routing e2e for channels, DMs, threads, legacy links, stale caches, and first-workspace creation