fix: prevent screen blink when switching channels

[raaymax]

Summary

• Replace full-screen isLoading gate with isInitialLoading that only blocks UI during first app.init() — not on every channel switch
• Channel fetch (getChannelById) now runs in the background using app.setLoading() for inline spinner, keeping the layout visible
• Removes debug console.log statements from the route handler

Behavior changes

Scenario	Before	After
Switch between known channels	Full-screen "Loading..." blink	Instant — layout stays, content swaps
Navigate to uncached channel	Full-screen "Loading..." blink	Layout renders, inline spinner, then content
Navigate to non-existent channel	Full-screen "Loading..." then error	Layout renders briefly, then error page
Initial app load	Full-screen "Loading..."	Full-screen "Loading..." (unchanged)

Test plan

• Switch between channels — no blink, layout stays visible
• Hard refresh — full-screen "Loading..." shows until init, then channel renders
• Navigate to /#/000000000000000000000000 — error page appears after brief moment
• cd app && npm run types — no new TypeScript errors