feat(ui): scaffold internal/ui with Writer + theme + mode (#74 PR1)

[dipto0321]

Summary

Scaffolds the new internal/ui package and migrates nodeup version
to use it, closing #74 PR1 of 4. PR1 establishes the Writer surface
(theme + mode decision + plain/fancy implementations) so the rest of
the migration has a well-defined dependency to move call-sites to in
PR2 (spinners), PR3 (interactive prompts), and PR4 (the remaining
direct-print sites in upgrade/check/list/packages/config/report).

What this PR does

1. internal/ui/mode.go — new Mode type
   (PlainMode / FancyMode) and a single DecideMode(noColor bool)
   decision point. The rules: --no-color flag → PlainMode, NO_COLOR
   env → PlainMode, non-TTY stdout/stdin → PlainMode, otherwise
   FancyMode. TTY detection is centralized here (a small
   isTerminal helper around os.File.Stat() checking
   os.ModeCharDevice) so lipgloss's own detection and our gating
   never disagree.
2. internal/ui/theme.go — Theme struct holding 6 lipgloss
   styles (Success, Error, Warning, Info, Dim, Heading)
   and DefaultTheme() constructor. Colors deliberately muted —
   nodeup output is informational, not decorative. The accent
   colors (success green, error red, warning amber) are
   AA-readable against the standard dark terminal background.
3. internal/ui/writer.go — Writer interface with 6 methods
   (Success/Info/Warn/Error/Println/Mode), NewWriter
   constructor, and two implementations: plainWriter (every method
   is a single io.WriteString, no decoration) and fancyWriter
   (each method renders the message with the corresponding
   Theme style). NewWriter forces
   lipgloss.SetColorProfile(termenv.TrueColor) in FancyMode so
   ANSI escapes are emitted even when stdout isn't a real TTY (the
   test-only escape hatch the package's tests rely on).
4. internal/cli/root.go — PersistentPreRunE now resolves
   the active ui.Writer once per invocation (calls
   ui.DecideMode(--no-color) and constructs the right Writer) and
   stashes it on cmd.Context() under an unexported
   writerCtxKey{} key. Subcommands pull it out via a
   writerFromCmd(cmd) helper. warnInterruptedUpgrade now takes
   the writer and routes its hint lines through w.Warn.
5. internal/cli/version.go — the PoC migration. Every
   fmt.Fprintf(out, ...) becomes w.Println(fmt.Sprintf(...)).
   The byte-level output shape stays identical (version.go calls
   Println which renders raw bytes in both modes), so the
   existing TestVersionCmd_* tests pass unchanged.
6. internal/cli/version_test.go — added
   TestVersionCmd_UsesInjectedWriter which constructs a full
   NewRootCmd(...) (so PersistentPreRunE runs) and exercises
   nodeup version end-to-end, confirming the Writer is correctly
   threaded and the PlainMode output is byte-clean (no ANSI, no
   glyphs).
7. CHANGELOG.md — entry under [Unreleased] / ### Added.

What this PR does NOT do (by design)

• Spinners — bubbletea integration into upgrade.go's
  long-running steps is PR2.
• Interactive prompts — huh migration for cleanup.go's
  y/N + any manager-picker is PR3.
• Remaining migrations — check.go, list.go, packages.go,
  config.go, root.go (the rest), and the final report.go
  summary block move to the new Writer in PR4.
• bubbletea/huh deps — kept out of go.mod for this PR. They'll
  be re-pulled by go mod tidy in PR2/PR3 when the spinner /
  prompt methods are actually used.

Verification

$ make ci
go mod tidy
gofmt -s -w .
goimports not installed, skipping (run: go install golang.org/x/tools/cmd/goimports@latest)
go vet ./...
golangci-lint run ./...
0 issues.
go test -race -coverprofile=coverage.out -covermode=atomic ./...
	github.com/dipto0321/nodeup/cmd/nodeup		coverage: 0.0% of statements
ok  	github.com/dipto0321/nodeup/internal/cli	1.612s	coverage: 40.1% of statements
ok  	github.com/dipto0321/nodeup/internal/config	(cached)	coverage: 81.0% of statements
ok  	github.com/dipto0321/nodeup/internal/detector	(cached)	coverage: 77.5% of statements
ok  	github.com/dipto0321/nodeup/internal/node	(cached)	coverage: 71.5% of statements
ok  	github.com/dipto0321/nodeup/internal/packages	(cached)	coverage: 37.4% of statements
ok  	github.com/dipto0321/nodeup/internal/platform	(cached)	coverage: 55.6% of statements
ok  	github.com/dipto0321/nodeup/internal/ui	2.097s	coverage: 67.7% of statements
total:									(statements)			62.2%

Coverage went from 59.6% → 62.2% (the new ui package brings 67.7%
on its own, and the cli package climbed from 31.1% to 40.1% because
the new TestVersionCmd_UsesInjectedWriter covers NewRootCmd's
writer-wiring path).

Refs

• Closes feat(ui): implement internal/ui — colored output, spinners, and interactive prompts via the Charm stack #74 (PR1 of 4)
• Builds on: docs: CLAUDE.md known-bugs section and phase-status table are stale #54, docs: post-upgrade cleanup docs inaccuracies (wrong issue ref, fabricated nvm-windows note, false PATH failure mode, Phase 7 status conflict) #60, chore(cleanup): dead code + fnm error-wrapping + nvm Current() test gap from cleanup-feature PR #61, ci(release): missing --provenance on npm publish; third-party Actions pinned to mutable tags not SHAs #67, docs: CI/release tooling doc-accuracy batch (CoGitto check doesn't exist, 6 vs 5 binaries, make ci description, coverage filename) #69, refactor(detector): Manager interface lacks context.Context parameter on Version/ListInstalled #53 (all merged)

[cocogitto-bot]

✔️ c207a05 - Conventional commits check succeeded.