fix(server): reload kwelea.toml on change during serve

[engineervix]

Summary

• rebuildAndReload was reusing the in-memory *config.Config loaded at startup, so kwelea.toml edits (docs_dir, output_dir, base_url) were silently ignored until the server restarted
• Extract isConfigFile and reloadConfig helpers (independently testable)
• watch now detects config file events, reloads from disk, re-applies CLI overrides, then snapshots currentCfg into the debounce closure to avoid a data race
• Bad TOML is logged; the previous config is retained — server keeps running
• server.Start gains an applyOverrides func(*config.Config) error param; serve.go passes a closure over applyFlagOverrides so CLI flags always win after reload

Closes #38

Test plan

• go test ./internal/server/... — 8 new tests all pass (isConfigFile × 3, reloadConfig × 5)
• go test ./... — no regressions
• go build ./... — clean
• Manual: kwelea serve, edit kwelea.toml, confirm "→ config reloaded" in logs and rebuild uses new values
• Manual: kwelea serve --source content/docs, edit docs_dir in toml, confirm CLI override still wins
• Manual: write bad TOML, confirm error logged and server keeps running

[juan-lorenzo-bot]

Code Review Summary

Verdict: Approved ✅

Verified locally on pr-39 against Go 1.25.7:

• go vet ./... — clean
• go build ./... — clean
• go test -race -count=1 ./... — all 8 new tests pass, no regressions in internal/builder, internal/nav, internal/parser
• Manual smoke test of kwelea serve:
  • Edit docs_dir in kwelea.toml → log shows → config reloaded then → rebuilt ✅
  • --source cli-docs continues to win over docs_dir set in toml after a reload ✅
  • Bad TOML → error logged, server keeps running, HTTP 200 retained ✅
  • Reproduced the bug on main: touching kwelea.toml produced no reload at all

💡 Suggestion (non-blocking)

addDirRecursive is called once at startup with the original cfg.Build.DocsDir. If a user changes docs_dir in kwelea.toml to point at a different directory tree, the watcher still only watches the old tree — so edits in the new tree won't trigger rebuilds (only the toml-triggered rebuild will pick them up, and only the first time, since addDirRecursive isn't re-run). This is pre-existing behaviour, not introduced by this PR, but worth a follow-up issue. The current PR is correct in scope.

✅ Looks Good

• Clean separation: isConfigFile and reloadConfig are independently testable
• The currentCfg snapshot pattern in the time.AfterFunc closure is the right way to avoid the data race
• applyOverrides func(*config.Config) error injection keeps the server package free of cmd-package dependencies
• Test coverage of the failure paths (bad TOML, missing file, override error) is exactly what's needed
• Doc comments on watch, isConfigFile, reloadConfig and the new Start parameter are clear

Reviewed by Hermes Agent (juan-lorenzo-bot)