Fix startup panic when Code Mode is disabled

[SAY-5]

Problem

VoidLLM panics at startup with a nil pointer dereference whenever Code Mode is not explicitly enabled:

panic: runtime error: invalid memory address or nil pointer dereference
github.com/voidmind-io/voidllm/internal/app.New(...)
	/app/internal/app/app.go:759
main.main()
	/app/cmd/voidllm/main.go:85

Fixes #87.

Cause

app.New builds codeModeService unconditionally and does schemaTTL: *cfg.Settings.MCP.CodeMode.SchemaTTL. But setDefaults only assigns SchemaTTL a default when CodeMode.IsEnabled() is true, so the pointer is nil in the (default) disabled case and the dereference panics.

Fix

Dereference SchemaTTL only when it is non-nil, falling back to a zero time.Duration otherwise. The Code Mode entrypoints on the service are not registered/reachable when Code Mode is disabled, so the value is unused in that path.

Test plan

• Added TestSetDefaults_SchemaTTL_NilWhenCodeModeDisabled in internal/config documenting that SchemaTTL stays nil when Code Mode is disabled (the precondition this fix tolerates).
• go test ./internal/config/ ./internal/app/ passes; go vet clean. (internal/app requires ui/dist built per CONTRIBUTING; with that present the package builds and tests pass.)

[snyk-io]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/app/app.go	0.00%	4 Missing ⚠️

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