feat(registry): support multiple plugin and theme registries via env vars

[louzt]

feat(registry): support multiple plugin and theme registries via env vars

Closes #2763.

The plugin and theme registries currently hardcode a single git repo
(registryRepo const, cloned into a single cache dir). This blocks the
common case of users who maintain a personal or community registry
alongside the official one. This PR adds multi-registry support driven by
two env vars and keeps the default behavior identical for users who don't
opt in.

What this PR does

• Both core/internal/plugins/registry.go and core/internal/themes/registry.go
  now hold []RegistryConfig{Name, URL} instead of a single hardcoded URL.
• ParseRegistriesFromEnv() reads DMS_PLUGIN_REGISTRIES /
  DMS_THEME_REGISTRIES (comma-separated git URLs, trimmed, empty entries
  skipped). Defaults to the official AvengeMedia/dms-plugin-registry.git
  when unset, whitespace-only, or all-empty.
• Update() iterates over registries, clones/pulls each into its own
  <cacheBase>/<name>/ subdir, and aggregates results in declaration order.
  First occurrence of each plugin/theme ID wins — duplicates from
  later registries are silently dropped, so a personal registry cannot
  shadow the official one by accident.
• migrateLegacyCache() runs once per Update() to move the pre-refactor
  flat <base>/plugins/ (and <base>/themes/) into the new
  <base>/official/ layout. Idempotent. Without it, every existing
  user re-clones the official registry on first Update after upgrade.
• loadPluginsFrom(dir) / loadThemesFrom(dir) extracted from the
  monolithic loader so each registry's cache is read independently.
• GetThemeSourcePath / GetThemeDir now search all registries (first hit
  wins, fallback to first registry) so multi-registry themes resolve to the
  right path.
• Per-registry errors are wrapped with the registry name
  (fmt.Errorf("registry %s: %w", cfg.Name, err)) so triage is one grep away.

Decisions taken

• Env vars, not config file. Matches the existing DMS_* override
  pattern (DMS_SOCKET, DMS_DEBUG); avoids introducing a new on-disk
  format for a feature that may not need persistence yet. A future PR
  could add ~/.config/dms/registries.toml for users who want named
  registries (louzt, nix-community) with the same semantics.
• Auto-naming r0/r1/... from index. Keeps the env var syntax to URLs
  only. If the maintainer prefers explicit names, a one-line parser change
  (name|url) is enough — happy to ship that as a follow-up.
• Declaration order = priority. First hit wins in both Update()
  aggregation and GetThemeSourcePath resolution. No disabled: bool
  field yet — adding it later is additive (defaults to false).
• official is the only stable name. When users set
  DMS_PLUGIN_REGISTRIES=https://github.com/foo/bar.git, the cache dir is
  <base>/r0. The official name only applies to the default fallback so
  users can layer a personal registry on top without conflicting with the
  default's stable name.

Scope boundary

This PR does NOT:

• Add a per-user config file (registries.toml) — see Decisions, follow-up.
• Add a disabled / per-registry priority flag — additive, follow-up.
• Change how plugins/themes are installed (still per-plugin JSON /
  theme.json discovery) — only how the source list is composed.
• Touch the IPC layer, the widget system, or the settings UI — those are
  separate concerns.

Validation

$ go build ./...
(no output)

$ go test ./internal/plugins/... ./internal/themes/...
ok      github.com/AvengeMedia/DankMaterialShell/core/internal/plugins   0.006s
ok      github.com/AvengeMedia/DankMaterialShell/core/internal/themes    0.004s

$ go vet ./internal/plugins/... ./internal/themes/...
(no output)

$ go test -run TestParseRegistriesFromEnv -v ./internal/plugins/...
=== RUN   TestParseRegistriesFromEnv
=== RUN   TestParseRegistriesFromEnv/defaults_to_official_when_env_unset
=== RUN   TestParseRegistriesFromEnv/defaults_to_official_when_env_is_whitespace
=== RUN   TestParseRegistriesFromEnv/parses_comma-separated_URLs
=== RUN   TestParseRegistriesFromEnv/trims_whitespace_around_URLs
=== RUN   TestParseRegistriesFromEnv/skips_empty_entries
=== RUN   TestParseRegistriesFromEnv/falls_back_to_official_when_all_entries_empty
--- PASS: TestParseRegistriesFromEnv (0.00s)
    --- PASS: TestParseRegistriesFromEnv/defaults_to_official_when_env_unset (0.00s)
    --- PASS: TestParseRegistriesFromEnv/defaults_to_official_when_env_is_whitespace (0.00s)
    --- PASS: TestParseRegistriesFromEnv/parses_comma-separated_URLs (0.00s)
    --- PASS: TestParseRegistriesFromEnv/trims_whitespace_around_URLs (0.00s)
    --- PASS: TestParseRegistriesFromEnv/skips_empty_entries (0.00s)
    --- PASS: TestParseRegistriesFromEnv/falls_back_to_official_when_all_entries_empty (0.00s)
PASS
ok      github.com/AvengeMedia/DankMaterialShell/core/internal/plugins   0.005s

Themes have parallel coverage (TestParseRegistriesFromEnv with three
subtests: unset, comma-separated, all-empty fallback).

Existing tests for TestUpdate, TestList, and TestLoadPlugins were
updated to the new method signatures; new subtests exercise the
multi-registry path end-to-end with a mock git client:

• TestUpdate/aggregates_from_multiple_registries — official + louzt, both contribute
• TestUpdate/dedupes_by_ID_with_declaration_order_priority — first registry wins
• TestUpdate/migrates_legacy_cache_directory — <base>/plugins/ → <base>/official/plugins/
• TestUpdate/no-op_migration_when_legacy_cache_absent — fresh install skips migration
• TestUpdateMigration/migrates_legacy_themes_cache — themes parallel coverage

Diffstat

 core/internal/plugins/manager.go        |  17 ++
 core/internal/plugins/registry.go       | 161 ++++++++++++++++++++-----
 core/internal/plugins/registry_test.go  | 212 ++++++++++++++++++++++++++++----
 core/internal/themes/registry.go        | 213 ++++++++++++++++++++++++++++-----
 core/internal/themes/registry_test.go   | 151 ++++++++++++++++++++++++
 5 files changed, 667 insertions(+), 87 deletions(-)

Three logical commits on feat/registry-list-config (master..HEAD):

1. 86f7550b — feat(registry): support multiple plugin/theme registries via env
2. de79ae8f — feat(registry): dedupe by ID + migrate legacy cache
3. ed7511ec — test(themes): cover legacy cache migration (parallel coverage)

Two packages, one logical change. Plugins and themes share no code; this
PR keeps that property (no shared registries helper package introduced
yet — easy to extract later if a third caller appears).

Backwards compatibility

DMS_PLUGIN_REGISTRIES and DMS_THEME_REGISTRIES unset → single
official registry, identical to the previous behavior. Existing CLI
flags, config keys, and widget bindings are unchanged.

Cache dir moves from <base>/plugins/ to <base>/official/plugins/
on first Update() after this PR. The migration helper
(migrateLegacyCache) renames the existing flat layout in place; if no
legacy dir is present, it's a no-op. This is invisible for users on a
fresh install and recovers a fast-forward pull for users on an upgrade.

Why this is the right shape for #2763

Issue #2763 asks for "support custom plugin and theme registries". The
env-var approach:

• Ships in one PR instead of three (config-file, naming, priority).
• Doesn't require a settings-UI change to be useful.
• Lets power users script it (DMS_PLUGIN_REGISTRIES=$WORK_REGISTRY dms).
• Composable with future PRs: a config file can override the env var,
  a disabled flag is additive, named registries are a parser tweak.