fix: respect OLLAMA_HOST env var for remote Ollama servers

[rishabhvaish]

Summary

• Fixes crash on startup when OLLAMA_HOST env var is set to a remote server address
• Cobolt now detects OLLAMA_HOST, uses it as the connection URL, and skips spawning ollama serve locally when the host is remote
• Provides a clear error message if the remote Ollama server is unreachable

Root Cause

When OLLAMA_HOST is set (e.g., 100.106.67.57:11434), ollama serve tries to bind to that remote IP locally, which fails with:

Error: listen tcp 100.106.67.57:11434: bind: The requested address is not valid in its context.

The existing code always tried to spawn ollama serve when the server wasn't reachable, without considering that the user may have configured a remote host.

Changes

Function	What it does
resolveOllamaHost()	Reads OLLAMA_HOST env var (supports both host:port and full URL formats), falls back to stored config
isRemoteOllamaHost()	Checks if the host is non-local (not localhost, 127.0.0.1, ::1, 0.0.0.0)
initOllama()	Skips ollama serve spawn when remote, shows clear error if remote server is unreachable

Closes #77

Test plan

• Set OLLAMA_HOST to a remote server that is running Ollama → Cobolt should connect without spawning local server
• Set OLLAMA_HOST to a remote server that is NOT running → Cobolt should show a clear error message about unreachable remote server
• Unset OLLAMA_HOST → Cobolt should behave as before (start local ollama serve if needed)
• Set OLLAMA_HOST=localhost:11434 → Should still spawn local server if not running (treated as local)

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