fix(cli): let theme show accept custom theme file paths again

[jongio]

Problem

theme show validates its argument against ListThemes() so a typo errors out
instead of silently printing the default theme (added in #404). ListThemes()
only returns built-in names, so that check also rejected the documented path
form:

$ grut theme show ./my-theme.toml
Error: unknown theme "./my-theme.toml" (available: catppuccin, default, gruvbox, tokyonight)

theme.Load explicitly supports path arguments and reads them from disk, and
docs/configuration.md documents the form. So a documented feature became
unusable, and the error message actively misled by implying only built-ins
exist.

Regression is from this unreleased window, so no released version is affected.

Fix

Skip the built-in list check when the argument looks like a path, and let
theme.Load handle it. A bad path now surfaces the real loader error instead of
a misleading list of built-in names.

looksLikePath is exported as LooksLikePath rather than reimplemented in
cmd, so the two places that decide what counts as a path cannot drift apart.

Verification

Confirmed end to end that the path form reaches the loader and reports real file
errors:

$ grut theme show /tmp/custom.toml
Error: theme "/tmp/custom.toml": missing color values: foreground, cursor, ...

Unknown bare names are still rejected exactly as before:

$ grut theme show grubvox
Error: unknown theme "grubvox" (available: catppuccin, default, gruvbox, tokyonight)

New tests cover four path shapes (relative, ./-prefixed, absolute POSIX,
Windows), that the argument reaches load unchanged, and that a load failure
propagates rather than being masked as "unknown theme". Full cmd and
internal/theme suites pass.